Basically I thought maybe you could have a co-op game where the second player can jump in at any point in a persons game taking the role of a standby npc, that works a bit like a follower. The hero of the game, the dovahkiin, nerervaine or whatever he/she will be called next time will still be the original player of the game and everything will be centered around that character.
The game would still be built with the focus on a single player experience. But at any time a second player could jump in to play in that world, as a less significant character, like a thane or something similar . If that second player disconnects the character of the second player once again becomes an essntial npc that will go back to where it belongs, it's home in the game world. And the original player can keep on playing on his own, much like what happens when you dismiss a follower.
The second player would still get to design his character and participate and could be mentioned by in game characters, but still be a little bit of a sidekick. For the sake of immersion, the second player could take the role of a newly moved in habitant in a city, or arriving by ship or, anything you like.
I think with this approach the game would still feel like a singe player adventure focusing on one main hero, and one persons personal adventure and story, but with the ability to invite a friend into it for a while. I have always loved the fact that tes is single player and I can play it at my pace and the game centered around my experience, but sometimes I do feel it could be fun to share the experience more, I feel this would be a nice compromise.
And yeah I realize it might not be as fun for everyone to be the side kick, but hey you could take turns taking that role in each others games .
What do you think? Do you have any other ideas or approaches?