Honestly, I'd rather them keep X-Men out of this. It's been a while since X-Men have really been featured in a game (UA3 is the only one they've been in recently) since Marvel was on a mission to make the franchise unpopular enough that Fox would sell the rights. Which now is no longer an issue... But, if they keep X-Men out of this, it will make the eventual X-Men game they show up in feel more exciting.
If they did characters known to be in the Avengers, I think it's fine and it'll kind of gateway into making an x-men game. We don't need Emma Frost, Jean Grey, or Cyclops in the Avengers but since Beast is pretty closely related, I think he would be fine.