I like your definition of replayability. I like grand campaigns more than 30 minute RTS style games as well and I always play Civ 4 on the larger maps. However, I think 2-3 weeks of hard playing is enough for a grand campaign. It's a reasonable amount of time to complete one and start a new one.

Pever, what about fighting a more defensive battle against one of your opponents? It looks like you're trying to counterattack and conquer them both. If you could fight more defensively against one of them and pull more resources to the other side, it might make something happen.