The last thing the Marvel Movie Universe needs is a bunch of mutants cluttering it up and acting as plot devices in stories they don't belong in. Aside from XMOW I think X-men is fine where it is and I hope Marvel does not get the rights back, at least not any time soon. Also, I'm glad that Marvel can't use Mephisto because he's become a lame plot device and I don't want to see Marvel using him in the movies when they should strive to have consistent and smart writing. Spider-Man I don't really care about right now, because I love the first 3 but am ambivalent about the reboot. I'd like to see Dr. Doom in an Avengers movie so I guess that the Fantastic 4 would be cool for them to get back, but overall I do not really support this "ONLY MARVEL SHOULD MAKE MARVEL MOVIES!" mentality.
Also, if anything Marvel should be trying to split the X-men comic universe away from 616. 616 is too crowded for the X-men universe to fit inside it, and the recent attempts to make it work have been very poor. 616 is stifling the X-men universe's growth, and the X-men universe is too big to fit inside 616. I think X-men should exist separately from the Marvel Superhero universe, because the X-men aren't really superheroes in the traditional sense anyway. It's always been more of a science fiction comic and I don't think that X-men and the Marvel Superhero universe really benefit from each other enough to justify all the problems that are caused by them sharing a continuity. I'm glad that at least the movies don't have to have that problem, since the X-men films exist in a separate universe from the Marvel Movie Universe.
Even Iron Man 1 did not reach the heights of X2 and XFC IMO, so I do not think reverting to Marvel would help the X-men franchise in terms of quality. As much as I disliked XMOW, and as much as many fans (not me) disliked X3, I think it's safe to say that on the whole Fox has done much more good with the X-men franchise than bad, and I do not trust Marvel to not screw it up by trying to shoehorn it into the Marvel Movie Universe.