I suppose the inevitable answer to that is Ba Sing Se. That would, I assume, make every season end with a giant seige battle: the North Pole, Ba Sing Se, and the Fire Nation capitol. There's symmetry in that, and we've had so many allusions to Ba Sing Se at this point, that it'd be a cop-out not to let us see it.

You know what that means for Zuko, right?

Every season will end with Zuko using his mad infiltration skills to break into an unbreakable fortress!