I agree with you that trying to merge multiplayer with singleplayer would be a very difficult affair. I think swtor attempted this, but I cannot say because I never played it.

I am excited to see if anyone takes up the idea of making something akin to the elder scrolls franchise and adding small scale multiplayer into it. Not specifically co-op but just where multiple people are dropped into the world together and then left to their own devices. In a game where up to say 10 people were allowed to wander free it would be fun to have rivalries and alliances arise as people try to further their own goals.

What saddens me is that this sort of idea seems to be forgotten completely in lieu of the industry direction that seems to suggest the only multiplayer fantasy rpg games we'll ever get are shameless wow clones.

I would love to be wrong by the way, if anyone does know of any of these sorts of games feel free to correct me.