What would you think of a multiplayer RTS, but where there is a vast ongoing struggle between various kingdoms and empires of the 4 races? (Persistent, vast MMO shared world.) The player doesn't command any side or any kingdom in its entirely, but can act as a hero unit and can command forces in his/her vicinity. (Basically, they're all like a kind of King Arthur who can reappear in a time of need.) The empires and kingdoms are entirely managed by AI and follow a cycle, where they reach a peak of their power (a morale stat follows a peaking curve) then decline and die.
The player works to increase his/her own legend by winning battles. The purpose of this design is to create an extremely complex and immersive world with depth of gameplay, but still enable players to "dip in" for a half hour session every once in awhile.