I think some kinda of super meta game would be cool.
However, I'm unsure about trying to cram a huge number of rounds into a small timeframe. This would make campaigns require dedicated time if you wanted to see it through to the end.
Instead, what about a kinda of global game that occurs over a few days. Several servers host a particular viable battleground. Players are registered to a central system on which side they've signed up for this "set". Games are played out on the different servers (adjusted # of servers based on number of players signed up and number currently playing). Enough wins over a certain period of time on a server triggers that one side has won the territory. Central map changes (hosted on a website somewhere) and new regions open up, maybe special unique powers, etc.
In this system, you can always have casuals come in and out and a "set" could take over a week, allowing people of different time zones to help out. However, if you set the # of victories to a lower number, you can have tight fast-paced single team matches where people may have to really plan how to allocate their players to keep the territories they want.
The tricky part is determining which territories to fight over. If you have a top level commander, if they win a territory they get to choose the next offensive. However, if they don't respond in a certain amount of time, that territory is opened up again for counter attack by the enemy commander to try and take it back. A strategic choice is then made to temporarily shore up other regions before opening up a new front.
Oh, and to prevent one player from being everywhere, there'd have to be a system that forces players to wait a set amount of time before they can re-enter a territory. That way, you can't instantly shift 10 players from one territory to the other, you'll have to have them "in transit" for a while.
This may be over complex, but in my mind a decent way to keep this manageable and to let the casual players enjoy it, not just the pro clans.