I think the villain of my current campaign should be trying to recruit armies from across the planes. Somewhere he's certain to find an army that'll work for cheap yet be effective. So the climax of the game will take place in a rather large war. The party will have seven or eight objectives -- some to protect people, some to destroy enemy resources, some to get in position -- before they can get to the main villain.
And somewhere there needs to be a planeswalker named Manuel. Manuel of the Planes.