I think this series is always doomed to mediocrity and I blame it completely on the fans.
I mean, even here in a hypothetical wishlist of how to improve this series, NO ONE is being ruthless about what was crap and what needed to go. This is why Activision releases the same game everytime, with different little trimmings and gimmicks, because fans will buy it regardless as long as it has their favourite characters in it.
I think this franchise needs to be completely redone, from the ground up. It needs to commit to it's genre, with proper RPG elements, proper dungeon crawler elements and proper, fun, beat em up combat, not a crappy, watered down version of all these things. Combat needs to be more than the same boring 3 combos (stun, pop up, etc) that have been in place since XM Legends and attacks need to feel unique to each character. Less static than just one generic block move and one generic dodge.
Instead of working over the fan service, how about working on giving people a game that is actually polished, deep and rewarding?
Has anyone played the Dynasty Warriors games? They are a deeply flawed series, but the one thing they have always done well is that feeling of just cutting and smashing your way through thousands of enemies. That's what I want, I want to actually FEEL like a superhero, not a river city ransom reject who is painted to look like Spider-Man. In my opinion, the games have NEVER made you feel like a superhero.
The closest I've come to that was in the very first X-Men legends game, when strong characters like colossus could break through walls, Iceman could make ice bridges and flying characters could pick up characters and things like that.