Gamestop is listing an "enhanced" greatest hits release on 8/7 for MUA. Unfortunately, I haven't been able to find any clue as to what the extra content is - more characters, I would guess.
Also, for those of you that already have the game, does MUA have the long loading times present in X-Men Legends? I had to give up part way through because I was getting frustrated with the time it took to swap out characters and/or open the menu!