We have discussed this at least 4 times PC. Here, and in other threads like the Apocalypse lounge. Marko just went over everything again. Do I really need to reiterate?
My bottom line is I don't need to see characters crossover who don't have much to do with one another's development. If I was to see X-Men characters cross with anybody, it would be with characters they fit more equally with. Much like you don't care if X-Men has been around for 15 years, rebooted F4, erased continuity and never established that other hero's have existed yet still think it's acceptable, I also don't care if they were to crossover with appropriate characters in the MCU. They have reality gems, future and past stories and all this other crazy sci fi ****. If the deal ever happened I would also be fine with X-Men being rebooted and re-cast since this would be at least 5 years in the future.
The whole idea of being forced into one another films is since Fox only have one option. F4 or nothing else. Otherwise we would never be having these debates. With the MCU, characters could be selected with whom they line up and for creative reasons. Not for being stuck with limited options.
Again, I don't need to see X-Men crossover into the MCU. I'm fine with Fox handling everything with X-Men alone. I prefer them being separate. But if any crossover were to happen I would rather it be with the company who has showed they know how to handle this stuff and own the better characters for X-Men to mash up with and can place them where they need to be. Plus it will have the benefit of these studios getting along. Which even with out crossovers needs to happen. So both these franchises can be more motivated to help one another.