Why?
I'm serious here. Most of the multiplayer crowd want multiplayer and singleplayer. But why make it an MMO? As someone who almost only ever uses single player, and the only conditions I do play multiplayer is hotseat on Civ4 or something, I don't really care about MM. And I especially wouldn't want a MMO.
I don't mean to offend, but I can see no reason for the idea at all. You yourself say that only a handful of 4x MMOs have been done, and none have been very successful.