I thought of an interesting take on a TDM recently...
There are three teams: the Allies, Axis, and Soviets. The map is linear, with the Allies starting in the west, the Soviets in the east, and Axis in the middle. The goal of the Allies and Soviets is to make it to the capture point in the middle - Berlin - before the other team. If they do, that team wins. However, the Axis is trying to prevent either team from doing so. The Axis is developing a super weapon that will require some time to activate; when time runs out in the round, the Axis wins. To make it more balanced, the Axis will most likely need stronger weapons, a better defensive position, or else a short round timer.
I think this would create interesting mechanics and team dynamic. The Axis will have to split their focus between two fronts, while the Allies and Soviets are adversaries but have an incentive for the other to do well.