I absolutely love co-op games. Here comes the copypasta.
We have triple battles now, so I was thinking that 1 or 2 friends could join you. Each battle would be a single/double/triple battle, depending on the amount of players. There would be Player 1 (The host), Player 2, and Player 3. The two extra players can do pretty much anything they can in their game, except advance their story. All players have freedom to roam the entire region at their heart's content, but they also have the power to summon everyone together, with the other player's permission of course. Maybe the feature could have a chat feature and be wi-fi compatible with friends.
Or they could just give us our dang Pokemon mmo. :3