What kind of co-op are you talking about? I can't see co-op working for a Pokemon game.
Kind of like a separate story where you and whoever you're playing with are rivals and you're to see who can become the champion. No evil team, non of the side stuff just a simple thing where you get all eight badges and see who can win first. Not necessarily co-op but a multiplayer. And you can only enter the multiplayer story world if you have the person you started with also going in (To avoid unfair advantage to the rival) I'm sure I could think of something better if I had time.
Would also be cool if it would stop you from proceeding to the next town until you vs your rival. Though I can see a lot of people not liking this idea.