Xenoblade Chronicles X + Online Cooperative multi-player = Possibly the best thing ever.

[Nuvendil]

I could see drop-in, drop-out co-op where you can drop in to a friends game and take control of one of the companion characters. Reason this is what I think they would do is to preserve the singleplayer story's integrity. And you know what? I'd be down for that. It would still be very cool because the big draw of co-op for this game specifically - for me at least - would be exploration in the enormous open world.