Lets take 4 Factions. (Think Daoc with 4 factions instead of 3..) Lets call them Albion, Hibernia, Midgard and Undead..
Put those 4 factions on a big square map with no instances. All Dungeons are open to anyone and all areas are completely unlocked. There is no leveling, gear is not a very important thing, 3 guys with the basics could take on one with the best gear for example.. skills get learned once then they become more advanced as you use them.
Now, Everyone on the server fights for control of this map and the map is roughly the size of all of WoW. all over the map there are mines, forests, lakes, ocean, all the basics, all these things however serve a purpose, Mines give you steel/gold/copper etc to make armor/weapons etc. forests give you trees for wood which can be used for weapons/siege equipment.. and just like any RTS game you can harvest and gather these things.
Use the wood you gather to build houses/barracks to better equip your individual guilds/alliances. Hire guards/peasents to gather these things for you aswell.
The basic Idea is it would be a MMORPG RTS In a huge 1v1v1v1 battle for control of the map with WAR/WoW game engine and servers that can hold 5-10,000 players. A Warcraft 3 MMORPG but instead of you controlling a bunch of little dudes, you are those little dudes and all you little dudes have your own minds :)
The question is, would you play something like this?