I said this in passing on Twitter, but the more I think about it, the better an idea it seems to be.
Part one: multiplayer Kerbal. A shared universe where you can dock at each other's space stations and so on. I think it'd be a lot more fun to build things if other people could see what you built. The complexity here is in the inherent asynchronous nature of the game, but I think you could use a combination of check-ins and flight reviews to handle that. So if I launch a rocket, then the other players can watch how it is going/how it went at whatever speeds they like, including allowing me to make notes (audio or text) explaining things.
Live multiplayer might also be possible and fun, with each player controlling a piece of the mission. For example, one player launching a rocket and the other player tracking it and beaming power to it.
Part two: multiplayer Kerbal with mods.
Here's the key: each player can have different mods installed. Think of it as each player being a different nationality, and each nation having slightly different technologies. If you want to change out your mods, just switch nationalities - or create a new nationality that has the mods you want.
Part mods tend to work very well with each other - there's not going to be a crash because you installed three different kinds of fuselage parts. But operational mods do tend to conflict and crash, at least in the older versions. I say - okay! Let's make that a part of the gameplay! If you have operational mods installed and they conflict then, guess what? The the technologies aren't compatible. One nation should have one, another nation the other! You can download "nation packs" of parts and mods as well as downloading those separately, allowing for a wider variety of customization. Nation packs could also alter things like texture packs, flags, Kerbin colors...
Multiplayer games could therefore be with each player playing a given nation, giving them unique capabilities. I might have those giant 5-meter launch arrays, but I don't have the enregy-beaming capabilities that my friend does. Maybe I launch one of his payloads on one of my rockets! I can't use his ship - that mod isn't active for me, so I can't take control - but, conversely, he can't take control of the launch stage. Cooperation!
Also, very relaxed asynchronicity as we need it. I could simply hit "pause" on the mission when I separate from his payload, then check in the universe. He would then be able to play that mission out visually, see my commentary, and immediately take control over his payload when he hits the end of the recorded mission.
Of course, you could do all the same things single player, but it wouldn't be as interesting...
Anyway, I like the idea. It's sort of like allowing players to have different tech trees, but with player-constructed mods as the "levels" of the "tree" instead of pre-planning it!