You could always have local co-op one one screen. While it isn't ideal, it has been done before. 

Yeah, it also stinks in every application it's been used, and those aren't massive open world RPGs. The framerate drops are monumental, and that's on 60fps games. Local isn'y happening.

It would be pretty awesome!
Definitely looking forward to it.

Champions of Norath and Final Fantasy Crystal Chronicles are sixth generation games that have same screen co-op and they did it decently enough. They're not open-world games, but having a person controlling another character isn't particularly taxing, in fact it would save resources through less AI. All you need to do is keep them tethered to each-other or you can just have one person control movement and another comes in during battles to assist. The hard part would be to coordinate movement within the battle, but that would be up to the players to decide how they want to do it. 

It's pretty simple you guys. You can play the game offline with the in-game "single player" party members, or you can play online with other "player characters" in your party. Same quests, same everything. Cutscenes will still show the standard party members. It's not complicated. There doesn't have to be exclusive online and offline content.