Getting the X-Men rights back would up the ante for Marvel Studios because then there would be a proper budget and a proper coherence/continuity. But would it result in the most mature and complex X-movies yet (FWIW I think even X2 is insufficient in this regard) or cookie cutter crossover fodder? We have to keep in mind that the social ontology (sorry for the high-flying wording here, lol) of X-Men is drastically different from anything else in the Marvel world. Whereas other "properties" only assume a handful of superpowered beings at any time with moral lines usually drawn clearly, X-Men assumes a biologically determined "epidemic" of supers amidst a "baseline" majority with a grievance/fear against/of them. This relation has plenty of potential for complex and thoughtful cinema which simply hasn't been explored yet (at least not to its absolute fullest potential).
It would seem to me that, given the unique nature of X-Men within Marvel, the movies should place a heavy emphasis on questions of identity, society, politics, philosophy and even existentialism. But also mutants as simply day-to-day persons and (some) a family, etc. Of course there will be action and these ideas can - and should - also be expressed through other means than pure dialogue, but just thought to throw that out there.
A completely new continuity in, say, a subdivision of Marvel Studios would give a greater freedom for this kind of stuff, but even in the mainline MCU it would be possible to integrate the X-Men, by, for example, a superhuman registration act / "Civil War" - style plot. Mutants would simply have emerged later than other superpowered beings. It could be explained by some hopefully nifty plot device. This way we could get a mixed flavor between the mainline MCU "family entertainment" ethos and something a bit more experimental and cerebral (no pun intended).
So yes, I think the X-Men and the vanilla MCU can mix to produce a tasty flavor
... even if it mightn't be exactly
what I wanted, but overall - overall
- it would be better than the FoX-Men.