What about the "Battlefield" game mode, where there are 2 teams, no attackers, no defenders. Its just war. 5 flags, 3 starting in neutral position, the 2 outer ones starting owned by each side. One could do like the original battlefield games, where each side has an amount of tickets and each death is a loss of one. However, I think the best is a 25 sec respawn time, and a timer on the round length. Also holding more flags (apart from the one you start with) gives +1 multiplier, and the team that manages to get all the flags wins the round and gets a +1 multiplier for next round. (though this would be lost if ones loses a round, just like normal battle)
Maps could be made up of different small centers on a pretty big map. There all class types could have their advantages. Some open fields between flags, some small castles/ruins with the flags on, villages, etc. I think this gamemode could be very very cool. This is after all the main reason for the Battlefield series' success. (rules could odcourse be changed from what I proposed, that was just the first rules I could come up with)