Me and my wife have been looking for a game we can play together for years and I almost thought skyrim was going to be it when I started to read info about you guys looking at co-op I was surprised you gave it up so easily. But I wanted to give you guys an idea on how you could manage a co-op that I think would be easy much desired by many who play your games.
Split screen option vertical or horizontal: players would be frustrated if they had to share a single hud screen, some players will prefer top/bottom to get more one view other will want side/side to be able to enjoy a different view.
The second player’s character would be imported as a Follower: most of us are not looking to do co-op for cinematic play we just want to play with our friends, and if I got to be his follower while all the cinematic stuff happened to him I would not care.
Special trading: I want to be able to give my friend equipment in the game, some potions, maybe a better weapon and not care if he takes it back to his game or not. But special game items should not be allowed to be traded, quest related items, one of a kind items (azures star, red hawk sword, dragon priest masks).
Game interactions: who ever is playing the imported character should play as a follower but with slight benefits, should they be able to talk to people yes, buy and sell from merchants yes, should they be able to enter buildings and leave town yes. But if one enters a building or leaves a town make sure it effect both, if one waits it effects both, if the main players dies it reverts to a saved game as normal. The second player should be able to interact with the world around him, pick up items, use the forge, talk to merchants but the game should still be based around the first player.
Questing: not required but nice, you could do this one of two ways, the first would be that the main character is the only one allowed to take any quest story & side, or it could be done that the main character can do any quest but the 2nd player is limited to the quests no one cares about, this way the players can still quest together. This would be limited to anything labeled a quest item. If first player takes a quest for a mammoth tusk then why can’t second player we all know there is enough to go around. But no everyone cares about the quests the main part is being able to play with a friend while still keeping balance in the game.
There are obviously other things that would need to be covered but I figured this would be most important for play.
Overall I think this would add that co-op everyone is looking for, it is not a huge change and those of use looking for a co-op RPG would finally have one.