MP would be pretty sweet, although a sort of P2P approach might be best (i.e. you can only be in one 'world' at a time, so more than one person can be at a someone else's base, but there's no way to travel to anyone's else's base without leaving your 'world' -- no separatae ) where one player is effectively the server and any other players are clients. Best example I can think is the way Minecraft lets you share the world you are currently playing in SP with other players, while you have the game open (everyone gets dropped if you quit /your/ game).
I wouldn't think it'd be that hard since you just need to show each player their own personal status and maybe an abbreviated set of bars/dots? for each additional player in your 'world'. Of course you also need to transmit the world data so they can render it and make sure they see the same air leak notifications etc. That stuff does add up pretty quick...
Just my two cents, but it might be easier to add now as opposed to later when their are even more features and systems to test for working in MP. Either way I look forward to seeing where the game goes.
