I'm bored with LotR Reloaded and I felt like tossing a couple of ideas out that I think are neat but let's be honest, are immensely unlikely to ever get around to doing anything with.. Go ahead and kick a few of your own out.

PvP (RvR) focused MMO - North and South
Alternate History. The northern states of the U.S. are building up their abilities with the clockwork and steel golem magics made possible by their industrialized infrastrucure and minerals. Filled with righteous fervor, they become increasingly offended by the summoning magics which the southern states use extensively for labor, and eventually, they go to war over it - Grant and Lee both commanding armies filled with mages and warriors of various sorts.
Structure: The northern and southern sections of the map are 'safe'; in between are contested areas and level-restricted battlefields. The edges of the 'safe' territory can shift if one side dominates. Magic users do almost everything through various pets and petspam - sort've like a DAoC designed specifically around animists, theurgists, necros, and summon spec spiritmasters.

A bit of a postmodern spin..
Tiny World
The Gods created the world.. simplified, and in a style like basic programming. The basic functions of the programming around which the world is built exist in physical form scattered across the globe. Magic users assemble their spells with heightened access to each of the functions, which they can quest to improve just as warriors can quest for enhanced abilities. What purpose was the world created for as hinted by the gods before their departure, and what will you make of your fate?

Tiny soldiery thing MMO
You're a little elf/ gnome thing, only an inch tall, in a modern full sized world, Run around houses, shopping malls, cars warehouses, and other assorted real life world, as you protect the unknowing humans from a tiny threat that might destroy the world! Oh, and don't let those giant feet crush you. Could have some fun puzzles, exploration and hidden cities inside walls and other assorted hidey holes.

I'd like to see a hybrid of Populus and Civilization in an MMOG setting where the citiznens are player characters. Basically you start the in the stone age, and as players work together they advance in technology. Meanwhile competing with other civilizations that are trying to do the same thing. Eventually reaching an end goal where the server would need to reset, rinse and repeat.

Aw man. That would rock! I'd work on something like that, if only it could be possible with tmmokit or RC...

ATITD is very close to that.