Demon ID said:
I would play as an Imperial Guard just to see if I'd be able to kill anyone.
Actually, it's theoretically possible to balance that. Like, allowing IG player characters to command a squad consisting of NPCs, making them an iteration of the summoner classes from fantasy MMOs. Although I've never heard of an MMO where you can shoot one of your NPC helpers in the face to make others fight harder. That, or making them able to hop into a tank like you said.
OT: I'd totally play an MMO based on Terry Pratchett's books. The problem is that you can't really make a modern MMO out of Discworld, because modern MMOs are too combat-oriented, with virtually no possibilities for roleplay (with LotRO and EQ2 being possible exeptions) and all non-combat activities being shallow and token.
Also, something completely devoid of any form of combat. And maybe grind. Like a game about colonising some distant planet, with players being the colonists who need to explore the world and manage their society, all wwhile maintaining the infrastucture of the colony.