Now that that's out of the way, if I could think of a single player game on PC that had a style of game play similar to that of an MMO I actually would be playing it. Skyrim is the only one I can think of and I'm honestly just not that fond of the game. MMO's tend to have a different feel to them than single player games even when you're looking purely at the core design. I like grinding, I like crafting, I especially like being able to talk to people when I'm just hanging out somewhere, and those things are sorely lacking in single player games. With the exception of crafting (which normally isn't very good in single or multiplayer games honestly) it's very hard to get a single player game that feels like an MMO, especially with the lack of ability to chat with a guild while playing a single player game.