This is a post to try and start a community project that has xenomorphs from the movies Alien and Aliens. I have been imagining xenos as a playable race for some time now. This first bit will be what I have thought of so far about how this mod will possibly work, or maybe some fun ideas we could have to go along with it. I have been thinking about this for this probably the past week or so. How we want to have a xenomorph mod that stays true to the films, but have xenos as an actual playable race, with a complete life cycle (depending on how much help we can get with the C# parts of this) with interesting and unique mechanics.
I have been doing nothing but tinkering with mods and learning how all of the xml stuff works over the past couple months and have what I think is a decent understanding of it. I've created a handful of my own personal mods that I'm using right now, and have helped a friend on their mod as well. I'm ready to take the next step and actually create something that I helped start and feel like I have contributed more. Anything I don't know or understand so far with xml I am willing to put in the time to learn. I'm willing to get the ball rolling with this project on all of the xml stuff. If I can get some help with it, especially for any C# and art, then I don't think this project should be too difficult with the frameworks already laid out for us. Of course everyone browsing this thread is more than welcome to make any suggestion, criticism, comment, snide remark about my mother, or offer of help in whatever way they can.
Just FYI, there already is an "Aliens" mod, but is nothing like what we want, no reproduction or life cycles or anything. They just act as animals or a hidden faction I think. A few days ago someone else just launched a WIP thread for a new playable race called Mantodeans, and damn, they look freaking awesome. But some of these ideas I had been thinking about for xenos I said in that thread as well, but they almost all originated from me wanting to have a
real xeno race with their reproduction cycle in the game. Also, I might borrow on some of the ideas that the author used for the Mantodeans, which I hope he doesn't mind (if so we will have to change these aspects!). Someone else just created a thread talking about a xeno mod asking about where to start with the concept, however they were only wanting to have xenos act as an animal, as far as I could tell. Not trying to step on anybody's toes, but it was this post that inspired me to want to make my own thread -
https://ludeon.com/forums/index.php?topic=39024.0 I had just spent a fair bit of time thinking about this, and felt that I had so much to say on the topic that it warranted it's own thread.
This probably won't be super cohesive or follow a specific flow of thought, I'm just going to be randomly throwing out these ideas as I remember them

We will use the term pawn interchangeably for the warrior and drone class xenos. We will also use hive interchangeably with colony.
I also should say that I am a pretty big "casual nerd" of the Alien franchise and it's lore. I've been watching Alien, Aliens, and Alien 3 since I was a child. The first movie scared the absolute pants off of me. After seeing that, and then going to Universal Studios them park in Florido (I think it was in Orlando, near Disney World) and going on the ride once, I refused to do it again. I was only six! However I loved watching Aliens for the action scenes. It wasn't until I was older that I could fully appreciate the first film, and also have a lot of fun imagining being a character on the second film. This isn't helped by Alien: Isolation. Honestly, I can't play that game. It is too perfect for me, aesthetically at least. I never even got to the parts where the xeno was hunting me all the time. Anyone who hasn't tried it, I
highly recommend at least checking it out. The developers of that game stayed completely faithful to the movies, as far as I can tell, and did an outstanding job. The ship interiors look exactly like they do from the films. It is ridiculous how much it all looks like the source material. I'm confident that the game has a very high Steam rating, and that it is a lot of fun. And do believe that I am
very critical of any games that I decide to invest my time into.
Anyway, Alien and Aliens are the only things that we will be basing any of this off of, as the rest in the franchise are abominations and we don't want to type an entire wall of our feelings behind any of these. The only things I will say about them, is that I adore Alien and Aliens, but Alien 3 turned to shit through studio interference (ever wonder why David Fincher doesn't have a Director's Cut? He won't put his name on it!) and Alien Resurrection was just holy shit Fox how could you let your IP be tarnished like this? AvP just wow. Prometheus I was originally excited for, but then when I saw it, it was not what I was expecting. It did give me hope that it's sequel could be an awesome bridge between Prometheus and Alien, but no. Mister Scott decided that he wanted to just shake up the entire Alien IP canon into an entire cluster fuck. I just want Ridley Scott to please go the hell away and make some other fantastic, but different, intellectual property.
The below section is assuming that we do not have any of the facehugger/human reproduction, and that a xeno actually comes out of eggs layed by the queen, not out of the belly of a human. If we could actually have the eggs hatch facehuggers, which would then need to impregnate a human (think of the grapple mechanic from the Vampires mod) who is either running around or wrapped up in the "cocoon" like Newt was, ready to be impregnated by the egg hatching right next to her. I suppose a good reason for separating, or making this project "semi-modular" is so that depending on the man hours that end up being able to be invested, we don't get in over our heads. We want to be able to at least get a base mod out. So we should also assume that any parts suggested in this thread could need to be put on an indefinite hold until we can create the opportunity for these features to be implemented. Also a few things might be a little redundant or I have mentioned them twice. That's because this is just a rough draft of me rambling my thoughts about this, going back and inserting things and moving stuff around,
trying 
to make the post a little more cohesive and easier to read. So this post definitely won't be perfect, but I hope you'll bare with me!

Also, I'm not claiming I know everything or am correct on 100% of this stuff and it being canon, but I have done a bit of research on the first two films, albeit this was a few years ago

-Xenos will be divided into classes. We will have a starting scenario with either a young queen by herself, or a few drones/warriors with a fertile queen egg that they must take care of.
-A queen will be the heart of a colony/hive. If she dies, and you have no fertilized queen eggs, then our hive should slowly wither away.
-The hive will wither away because xenos can only be healed from their minor wounds so much. This is because they have an exoskeleton that is difficult to repair, plus xeno doctors?

-You can only have one queen per colony.
-If you have a new queen hatch from her egg, or if there is more than one queen on a map, all queens will get increasingly irritated as long as they aren't the only queen on the map. Eventually a queen may get so pissed off she separates from her sac, and goes to attack whatever other queen she can get her claws on. The only remedy for each queen is to make sure they are the only queen on the map, whether the other queens run or get killed does not matter.
-Queens need to stay attached to their sac to keep producing eggs. If they break off from their sac (think back to Aliens when the queen had to essentially "rip" it off from herself so she could chase Ripley) then they might not be able to reattach it, maybe they have to grow a new one. If we did allow reattaching, then this should take a very long time or require a special surgery/procedure/something.
-Xenos do not care about losing other hive members, or this is very mitigated. Xenos get a mood boost for sleeping in a barracks with each others, and a penalty for sleeping solo, our outside of the hive.
-Xenos social moodlets will mostly revolve around the queen, and how well she and the hive as a whole are doing. The xenomorph is a selfless being, the hive always comes first.
-Xenos will never panic or run about uncontrollably (set on fire on fleeing from whatever). They always fight until the end.
-Xeno warriors can have a sort of "leap" mechanic to help them close in the distance to melee range, also because xenos have no ranged attacks.
-Xeno drones can still fight, but not nearly as well as warriors, nor do they have the ability to get the combat upgrades that warriors can.
-The main xeno workers are called drones, the soldiers are called warriors. There are a lot of other kinds in the Alien universe, but they aren't as well known nor as popular, and I also don't think they would really be needed - allowing the resources that would be spent on redundant classes to be better allocated elsewhere. I don't know how I would feel about having a ton of classes beyond these. I think it would be better if we just had these two main ones, but then we could have more specialized workers for different tasks. Same for warriors, some of them might have more health and better armor, while other's have less health but are a lot faster, etc.
-Xenos are easy to replace and sort of a "throw away" type of pawn. This is one of the biggest points of the mod and what I think will help it stand out among all of the other playable race mods. It will allow the player to think about all of their pawns differently, and
hopefully create a new and interesting ways to play RimWorld!
-Since xenos can only heal minor wounds on themselves, this is the perfect way to create a system of losing an acceptable amount of xenos when you have to engage in combat.
-The queen will have all sorts of various upgrades that she can receive to change everything about egg laying/production.
-To determine what type of xeno an egg will turn out to be, I can think of two decent sounding options. The first is all eggs are lain as generic, and the player can manipulate the egg environment or do something interesting to determine the class of the xeno egg, or the player can choose what type of eggs the queen will be laying.
-If we go with the generic eggs, then I think this could be a lot more interesting. We could have your drones put the eggs into "nursery" rooms, just like there were the egg chambers in the movies. Similar to how crocodile eggs work (and I'm sure a lot of others) you could have the average temperature that the eggs and incubated at determine if they will becomes drones or warriors.
-Maybe we have to do something very special for new queen eggs. Maybe they take a year or more to hatch after they've been lain. Say if they take a year, this will be a new system that you would have to balance with only having one queen per colony, but always wanting to have a back up queen egg that would be ready to hatch within a time frame that they could mature quick enough to start laying eggs before your hive is wiped out.
-Queens might take anywhere from a couple quadrums to a couple years to fully mature. Maybe 2 quadrums to mature, and another 2 quadrums to grow a new sac.
-Queens remain either completely stationary when they are attached to a sac, or they can only move within a few cells of where their sac is attached.
-The sac attached to queens has it's own health and anatomy, independent of the queen.
-The sac can be upgraded over time. However if a player detaches the queen from their sac, and if we decide to not let the queen reattach to a sac (must grow a new one) then the sac will wither away and die, making any investments into the sac go to waste.
-This will create an incentive for the player to highly consider where they place their queen and her sac, and if shit is hitting the fan in a hive raid, should they detach their queen or not? Will the hive survive without the queens help (she is an excellent big badass fighter)? Or if they
must use the queen to defend the hive, will she be able to survive after the fight? Will she be able to grow a new sac? Do you have enough eggs put up to keep your xeno population numbers up if you detach your queen? Do you have another queen egg that will be ready to hatch soon after if your current queen dies? So many possible dilemmas that will create great new stories every time!
-Queens could come from special eggs and special queen facehuggers (I think that I have seen these somewhere online before)
-Xenos sleep in/on/next to the walls, maybe gaining some type of camouflage or blending in to the wall. Think about in Aliens how the xenos ambush the marines, and Hudson famously yells, "They're coming out of the God damn walls!" Not necessary but could be a big bit of flavor if implemented properly.
-If we can have xenos hide in walls, that will be perfect for setting up ambushes for raiders coming in to attack your hive. If you have your hive/map sealed off in a specific way (think of how everyone funnels raiders to their killbox) then you could have your warriors set up for all sorts of interesting ambushes.
Below here is if we can find someone to help us with the whole facehugger impregnating a live human, which will then birth a chestburster (a baby xeno) of a specific class, or a generic chestburster that we can have "evolve" into a certain class as it goes through it's life stages. In the first Alien movie, it is inferred that the xenos have an extremely short lifespan. This is how the chestburster was able to grow so quickly, and why at the end of the movie for the final action sequence the xeno seems so slow and lethargic. That was because, as I remember believing after reading about it somewhere, the xeno had completed it's life cycle (in alternate cuts the xeno has Dallas and I think another crew member cocooned down below deck) and was just sort of settling in to die. However, in Aliens, it is clear that xenos can live much longer, but I am not sure to what extent.
-Xenos can cocoon any humanoid (basically anything that would have blood. So no androids, but cat people, dog people, star wars people, etc. should all be able to be impregnated) that they have captured as a prisoner. This could mean dropping consciousness down very low, or whatever it is when a pawn gets anesthesia for surgery, sedating them.
-There will be nursery rooms with eggs that are in their final stage, almost ready to hatch, where you can keep these cocooned prisoners.
-Once the egg hatches, the facehugger will latch onto it's victim (again, like the grapple mechanic from the Vampires mod) and impregnate it's victim over the next few hours.
-The facehugger and it's future egg surrogate will remain very vulnerable during these few hours, and anything that happens to them during this time could cause the impregnation to fail/abort.
-Facehuggers who fail while impregnating their host could die (not sure of canon on this, but sounds interesting)
-Other mechanics to let facehuggers or any of the steps along any of the egg/facehugger/chestburster/xeno life stages to fail, to help keep population numbers in check.
-Once a host is impregnated, they must be kept alive and healthy for the duration of the pregnancy.
-If the host does not stay optimally healthy throughout the pregnancy, there will be tiered chances of having a gimped chestburster/xenomorph.
-These lacking xenos might not have their handicaps be known to the player until the xeno is a full adult. This will encourage players to actually take care of their human hosts. If the player knows right away that a xeno will be gimped, they might kill the xeno right then in hopes of replacing it with a fully healthy one. However if they can not know this until they have at least invested some time into making the xeno a full adult, well then I think this would be a good incentive to help balance wanting to keep the human hosts healthy, both before and during pregnancy.
-If possible, or maybe through a new story teller if required, have new special events related to xenomorph population to help regulate whatever numbers are decided to be optimal for a hive play through.
-Right now I'm thinking that an optimal number would be around 30-40 xenos. With much of their social system stripped, and minimal variation between classes and their various upgrades, this should help balance out the extra load from the extra pawns on the map.
Stuff posted below here could be some fairly significant changes to Alien canon, but this is so that the IP can be adapted as best as possible for fun and interesting game play. Maybe also some significant changes to pawns/RimWorld to make all of these ideas work together.
-Xenos will have an increased XP rate for their skills, like burning passions, or they will have a lower cap on the maximum level for their skills. This will be needed so that the player will never be too attached to any specific workers. In fact, none of the workers or soldiers should significantly stand out from their peers. The only exception to this, that I can think of, should be for any major changes that a xeno receives via "carapace upgrades"
-Xenos can't wear armor or have weapons. Instead their combat prowess and stats will be determined by their melee skill, class, and "carapace upgrades".
-Xenos can go into a special "cocoon" for a duration of time to get a thicker shell/skin/armor, reducing damage taken. An example upgrade could be that their tail is longer, so that they have increased "melee reach". You can of course have some regular old boring plain damage/armor upgrades as well, but I don't think these should be too significant, ever. Remember, if the player can invest too much in any
individual xeno, then they stand the chance of becoming attached to this pawn, and this will go against the sort of "playstyle" that the mod is trying to achieve.
-Various colony wide upgrades, that come from special research, with great flavor names/descriptions that make sense. These researches will require that you have made certain buildings/things X amount of times throughout the colony, just like in vanilla some researches require you to have built a multianalyzer, stuff like that. The mod SS Researchable Stat Upgrades has a dll in it that allows this type of colony wide research, it can change a thingDef's comps or verb properties, so this is achievable. Maybe Spdskater would be kind enough to let me use it in the mod

-Thinking back to StarCraft, the Zerg would have something like a "spawning pool" that just sort of sounds like it made sense for them. There could be buildings similar to this that you should place throughout the colony. The reason for having these buildings could be simply for aesthetics, to make your colony actually
look and feel like a hive.
-If I had a talented C# programmer, then maybe we could have a sort of thing like there was for the Zerg in StarCraft or the Undead in Warcraft III. A building that spreads a sort of "blight" within so many tiles around it. This "blight" could then be required to place any xeno structures. Only a handful of buildings could be built on any tile that doesn't have this "blight" on it. And of course we would need a better name for blight.