Natural Caves/Abandoned Mineshafts

[Goldenpotatoes]

Mountainous regions cover large sections of the map with rock and the occasional ancient ruins stuck inside full of mechanoids who really hate miners. Its neat and all but its lacking fairly largely in what could be done with the 'underground' segment of Rimworld.

Ruins are already scattered around most maps, either it be just walls or actual buildings, its fair to assume at a point, people lived here before you crashlanded a long time ago. Hell, even most ore deposits you find are the result of long-buried buildings/ect being compressed together for scavenging. It's not a stretch to assume you'd occasionally find whats left of a mineshaft or cave. Prevent it from popping up initially by blocking it up with rubble and letting the player find it on their own, like surface ore deposits. Pop it open and explore what waits inside.

Natural caves are fairly iffy, as that'd be a pretty big buff to mountain bases, but hey, odds are if you're playing on a mountainous region, you're already planning on building a mountain base anyway so it only slightly speeds up the hallowing out process. This, combined with abandoned mineshafts, would give hive infestations more places to naturally spawn that isn't your base. There really isn't anywhere else on the world map that has 'overhead mountain' naturally, so its pretty much guaranteed that your mountain base is ALWAYS going to be the primary spawn location of these things, forcing you to act fast or somehow build around it. Having other spots for them to spawn that isn't an immediate player threat would allow more interesting gameplay in terms of forcing the player out of their protected colony or letting it grow into a decent size.

[Limdood]

Natural caves are fairly iffy, as that'd be a pretty big buff to mountain bases,

not if the caves could have pools of water in them :p

suddenly your base plans are ruined when there's a 8x13 lake where your freezer and dining room were going to be!