Here's a few more gametypes, best suited for 20-50 people total, ther all from Tribes 2
Hunters- Kind of like DM, however u don't get points just for killing. Every kill yields one flag which sits on the dead body until someone collects it. The only way to score it to bring one or more flags to the center of the map (in T2 it was called the nexus). The more flags you bring to the nexus the more you score for each flag.
Team Hunters- Same as above, but players' points count toward their team total
Rabbit- One flag sits in the center of the map at the beginning. The object is to grab and hold on to the flag for as long as possible while the rest of the pleyers try to kill you. Every second gave you one point plus one point for grabbing the flag in the first place. You loses the flag when you die, someone else picks it up, chase begins again. Somehow i think the only reason it worked it because you had jet packs that added a third dimention to the fight and you could shoot the flag and make it jump like 10 feet. It was fun as hell though with like 20 players.
Bounty- One of the best
Everybody gets to be a bounty hunter.
Match begins, on your screen it tells you what player to kill (randomly selected), you get no points for offing someone else. The only catch is that the other player is also searching for you! Whoever gets the kill first gets the points for that bounty, and the computer selects a new target for each of you.