PC COOP Game with Skyrim's Atmosphere.

It's late at night and because I was so impressed with the results of my last post, I thought I might as well bounce this craving off you guys before I hit the sack.

Basically, I want to play a game with a world as atmospheric and deep as Skyrim's, but with the convenience of being able to have a couple of people drop in/out. The game itself doesn't need to be an RPG-- it could just be an exploration game or the like. It just needs the atmosphere.

For those of you who saw my previous post, it's worth stating that this is completely unrelated to the types of game I was looking for a few days ago.