I looked through this tread again and have some new thoughts:
I think rounds (like nazi zombies or the gears of war survival) would be best with 10-20 seconds between rounds (think terra volta respec, the waiting added suspense). I think the map should be very small, smaller than the asteroid farm map so you wouldn't need to worry about hiders. I've been told (no idea if its true) that a cut scene allows the game to "reset" the map. If so, maybe that would cause less stress on the server? I would love to see most/all enemy groups involved, and maybe some custom ones. Just adding levels/buffing bad guys or putting players under constant debuff wouldn't be as interesting as different enemy groups that may require tactics change.
I'm done until I think of something else.