have you thought about creating your own campaign. you can take elements from other systems to make a campaign that fits what your wanting to do. which can work wonders later on when your group wants to do more campaigns
one that comes to mind is to have your master run around on a regional map trying to gather a few items to "rule the world" but the other players are trying to do the same thing and will eventually fight one another for control of said object
and each object obtained will give you a slight advantage that can be used in each game