Capture teh flag would be awesome, its the only game mode i play on halo3 Lol
same and free for all capture the flag would make it interesting this is how i think free for all CTF would work:
one flag, that spawns at random points (via flagspawn like solospawn and teamspawn)
players are all against each other for the flag then who ever has the flag must hold on to it as long as they can before they are killed, which accumulates points (so pretty much players don't get points via kills it is done in time increments which are decided in game room) and you have a time limit to get the flag before it spawns to another location