A few suggestions
-Being able to vote for "scrambling" the teams could be a great addition. If the vote passes, the teams would get mixed up before the next round starts, to even the odds when the teams are unbalanced. Maybe it shouldn't be random, so that for example the players with the highest scores would be in opposing teams.
I've found that several times when playing online, the teams are simply unbalanced because the "skill" of the players varies so much. But oftentimes the players hesitate to change teams, or the player numbers are actually balanced, which makes a manual balance rather hard.
-As grenadier mentioned on IRC, you often have to wait during the very first round of a map, for example in Bank Robbery or Round Team-Deathmatch, if you don't join a team very quickly. One idea is that every player automatically joins a team when the map starts, but I think it might be better if the start of the first round is simply delayed a bit.
-Inane mentioned this on the TIGSource forums: A Last Man Standing/Survival game mode. This would basically be Round Deathmatch, meaning that whoever dies in a round remains dead until the next one, and the last player still alive wins. I think this would be pretty cool, as it would start out as normal deathmatch but everybody has to be more careful, and in the end it would boil down to a duel