Level doesn't end immediately when the first player reaches the exit. It will just activate some timer and allow other players to get to the exit too, or get more kills and stuff. The first to reach the exit can also return to do all that. The levels would need a par time. Reaching exit before the par time will give bonus score. First, second and third gets some bonus score too. It will also have a base score, so the fourth one who reached the exit after par time would still get some score.
I think the levels could also have some pie, cookies, cake or whatever stuff that gives score. These items wouldn't respawn or stay there for everyone to pickup (like weapons, etc). Only the first one to get the pie will get the score. edit: the score from these items will be calculated when the level ends...
Killing other players would give score based on what weapon was used to kill and what weapon the other one had. The dead player would respawn to the start of the level or maybe some respawn checkpoint. Killing another player would take some random item (including pie) from the killed and spawn it as a pickup. I think it might feel a bit too unfair to get all his possessions. So the killed player keeps everything else he had collected so far. Of course killing someone enough times will give you all of his items.
Killing monsters could give score similar to players. What weapon was used against it and what monster it was. I think just hitting a monster or player could give some score too. Then a frag stealer won't get all the score, he would just get the score from the final hit (gib the monster/player for some bonus), which could have a bit more score than non fatal hits.
When the level ends, the highest scoring player wins. Then go to the next level. The level ends when everyone has reached exit or when the level end timer is zero.
Anyone done this kind of thing in a game or mod? Personally I think it sounds like fun and I'm going to do it for my mod.