Would it not be possible to have an MMO that lets you be something other than a single character?
For example, you control the development and advancement of a 100 man strong army. Forming alliances with other armies to do battle with opposing forces. XP would be garnered in similar fashion to standard MMO's but the scale would be huge.
You would decide the specialty (ranged, melee, rear support like medics or researcher teams, crafting) etc of your "army" just like you would in a character creator.
To make it more personable and less RTS like there would be one "captain" of sorts who would be the character through which you would command.
Huge allied armies 1000 strong controlled by 10 players taking on the same number of foes on a wide open battlefield. If you lose soldiers in the field of battle it costs XP to get back to full strength and if your "captain" bites it it's a huge XP hit.
It wouldn't be just PvP. There would be rogue bands of NPC barbarians and such roaming the lands that would need to be dealt with. Missions would include razing of villages among other things more standard to MMOs.
There could be crafting "armies" who would peddle thier goods to the players leaning towards combat and alliances would be made as such. The economy would be driven by warfare and the wares needed to wage it.
I could go on and on but it sounds cool to me. Oh, has anything like this been (or is being) done?