The basic idea is that there is a path which each team is trying to go down. However, they can not go any further until some condition is reached. Maybe the game could compare the amount of kills each team has had recently, or there could be an ball each team is trying to get to the opponent's side. Also, there would be a neutral zone. A map example at the start of a game might be something like this;
Later, it might look like this.
And at the end, it would look like this, at which the blue team could get the ball through the goal.
Of course, the path would be more convulted, with hiding spots, and other stuff.
The darkly colored =s are spaces the opposing team can not go into, no matter what. These areas would probably be spawn points, to keep down the spawn camping.
Come to think of it, this is a bit like the Bombing Run mode of Unreal Tournament, but one goal consists of an entire round.