My only issues? Making sure that such a game is balanced for both co-op, as well as single-player.
As great as a number of Co-op games, past and present, are, some among that number started to falter a bit when you just wanted to go it alone, for one or several reasons. Some of these same types of gripes even apply to RE5, based on what some impressions I've heard/read...
[spoiler]...especially when it comes to the final boss fight, which turns into more of a chore than the dynamic finish it probably should have been. But that's for another time and post...[/spoiler]
So, while I'd like to see such a thing happen to a Rockman game, I'm still wary of the notion that, in efforts of trying their damndest to focus on co-op, the single-player component (which should STILL be the bread-n-butter of any action-platformer Rockman game) suffers on the other end. And I wouldn't want that, either.
...Still, such a prospect, like being able to create your own "Rockman" avatar, and then team up to clean house with your partner? That WOULD be nice...if thought out correctly.