Imagine an MMORTS, I think Supreme Commander would be the best base for it, since it already uses immense areas of land and large scale tactics.
You could have several factions battling over entire planets for resources and strategic advantages. You'd start as a low level commander, fighting over small less important areas and have access to low tier units and buildings, then as you gain battlefield experience you increase rank and fight over increasingly larger more important areas with more powerful units. You could even have commanders who decide where to post lower ranking people and decide how much starting resources they should need to battle over over areas. Then you'd have the really high ranking players who'd be deciding what planets to conquer and building experimental units over long periods of time for use in major conflicts...