It depends on how flexible the engine is, (I have BIG ideas).
Like if it was a modern FPS engine with good physics, I would like a level which changes the way the deathmatch plays out while you play it.
So if you were in a shootout in a giant aquarium/underwater lair, if someone uses a rocket launcher and launches it at a weak point of a glass wall, the arena begins to fill up with water. Dangerous fishes begin to flood into the arena and start attacking the players. A timer could start, and then the fight turns from a fragfest, into a battle, to escape the arena.
Of course a system could be activated to reverse the effects, but you would have to take down the exotic fishes in the water in order to activate the machinery to repair the wall.
In effect, the losing team can reverse the situation and win, but the frags still count, so if they do it too late, they may end up losing anyway, (as the winning team could just finish them off). The winning team would have to decide to either capture the area where the escape route is and hold it off from capture long enough to escape, or they have a last ditch effort to kill the enemy team before they escape.
EDIT: Of course, an additional strategy for the losing team, is to blow the hole, and then set up a trap for anyone going to mend the wall. Its another possible way of catching up.
In essence, the game becomes as much about strategy, as it does teamwork and skill.
Unfortunately, I'm an ideas man rather than an actual do-er, (I can't get my head around coding, and mapping, and the third dimension scares and confuses me!)