I'm sure this has been asked/suggested over a hundred times before. I understand the issues that would come when implementing this, it's actually a decent amount of issues, but I feel if each was addressed individually, we could solve this conundrum and bring a stable system to the core game. Here goes...
Issues:
The game has time warp abilities, multiplayer would have to be ran off of the same time period. This means that time modification effects such as time warp would need to be disabled. I'm sure many could guess why that is an issue, without time warp, progression would seem slow and annoying.
If we use the same time, then colonies would have to be started at the exact same time in order to for said colonies to interact.
Possible conflicts with other colonists, two users targeting a single colonist for different actions might cause issues.
What has been done:
Global chat- Seems fairly neat, would be helpful for tips and new players, but would require servers. A mod has already done this, so it's possible.
Trading- A mod has done this. Sending resources to other players, I feel if properly configured this could be stable and yield interesting results. I'd say for trading you would need launch pods and such.
What would be liked:
Sending caravans to other player's colonies.
Status with other player's colonies.
Raiding of other player's colonies.
Visiting other player's colonies.
Basic interaction.
Possible global chat.
Trading.
Solution?
Just some basic ideas:
Disable time warp when playing online, but have an offline mode.
Make online servers where you have to start new on a pre-selected planet shared by other people.
Allow contact between different rimworld's via spaceship.
Allow the transfer of colonies to other planets/servers.
Let time be ran by players and say "whatever", but when interacting with players as in they are visiting or raiding, disable time warp.
That's basically it, I'm sure there are many more solutions.
Main issued would probably be server funding, but they could just use steam for coop.