I wish fallout 4 had co-op. I need I understand how in some aspects it would take away from the exploration and dialogue but when you've completed the campaign, your pretty much stuck with a giant sand box by your self. It's just no fun. My idea is that once a player has completed the main campaign they should be able to unlock co-op. Players can have fun and unique experiences in co-op. Haveing more frequent settlement attacks and a range of random events once multiplayer is activated. Something that will make the game appealing to more people. I mean I love fallout 4 but when I'm playing by myself and I've gone through just about everything and collected every gun I want, it's not to fun running around the map trying to annihilate every enemy, then sleeping for 2 weeks just to do it again.

If I played co-op with someone, would it be in my version of the commonwealth or theirs? Or a third "open" one which has no bearing on mine or theirs?

Because of how specific and different everyone's commonwealth is, it would be impossible for you and a friend to exist and interact with the game in the same world.

We have a thread for discussing multiplayer in Fallout games: http://www.gamesas.com/topic/1581454-fallout-multiplayer-discussion-thread/page-2