I don't know if an MMO style approach would fit well, but I am extremely interested in some sort of campaign multiplayer. Multiple players with their own parties/clans, joining whichever factions they wish, it would be exciting for sure.
The logistics of accomplishing something like that would certainly take a huge amount of work, and from what I've seen it is not in the criteria for release, so I imagine that modders would be able to make this a reality once the game is complete and mod tools are released. (Modders have apparently figured out how to make Skyrim multiplayer now, so who knows)
The immediate pain point I envision from a player perspective is menus. Would the game pause for everyone if a player opens a menu? If not, would the game pace be too fast? Too slow? If I'm trying to make a profit from trading looking through all the items in a town, buying low and selling high while another player lays siege to a settlement, it might be frustrating. How about combat? If one player is in the midst of battle, would time progress normally on the overworld? Certainly something to discuss, I would play the hell out of something liek this, even more so than I already am!