(pre-baked nav meshes, also!)
I'll probably be giving myself a real headache with this, but I'm working on procedural environments generated at runtime ala roguelikes, although there might be some savings from assembling chunks of geometry almost like 2d tiles.
All followers without jetpacks become grouped into one sub-party, and get their own pathfinding call.
Sorry I really should have been clearer, as mentioned above I wasn't just thinking of solving pathing. If you come to a big door and you need the demolitions guy, or you come to a chasm and you're the only one with the jetpack, which is the better design: Let followers just teleport to where you are, or make the player deal with not being able to pass the obstacle (or one of the other resources that might break down to the same thing essentially).
Teleporting is cheap, and I don't really like it because it doesn't fit the milieu.
Blocking the player from taking a follower, especially with a randomly generated environment, means either the follower is left behind or the player has to wind their way back out of the level (potentially through multiple floors) in order to address the challenge by re-equipping. I don't know if that's horrible, as it may force players to haul lots of crap to deal with variable contingencies. That could be good (balance say looting vs. equipping) or bad (micromanagement).
I think not breaking stealth is quite important, maybe they try to stay in cover if you're sneaking, and the enemies ignore them unless they do anything too blatant.
Something like stealth might have to be solved with level design. Maybe there are set challenges and clearing rooms where followers can gather. You could take a few, deal with some sentry or trap or whatever, then summon the rest of the followers onto the next room. It would probably lead to needing lots of abandoned areas, like empty barracks or storage areas, which could be okay I guess.