Yeah but I don't want to scrap the entire X-Men cast. I feel like we got a better Cyclops and Storm this time around and I don't want to lose Sheriden and Shipp. I also feel like Michael Fassbender and Ryan Reynolds are great as Magneto and Deadpool. Even if only Deadpool were added to the MCU, that's still in a different continuity than Legion. it isn't worth recasting the X-Men.
I'd rather see a story about time travel and alternate universes since A. It's the easiest way to bring Quicksilver back to the MCU and B. House of M and Secret Wars could be adapted. It isn't like there aren't storylines already existing which Marvel could use to have the Avengers and X-Men meet up in a way which retains both casts.
The only reason why it's tricky is that it needs to be an event film that both studios need to build up to. The FF are easier to bring in since there's no overlapping character like Quicksilver nor is there any ongoing FF film after Fox's second franchise got canceled. I really feel like the Fantastic Four are going to come to the MCU first and then Phase 4 is going to end with some sort of event that merges universes.
Ryan Reynolds can stay his movie isn't overtly in the X-MEN timeline, it can easily be added into MCU canon, hell even Ryan Reynolds himself said that Deadpool is not affected by the DOFP timeline. If Marvel is using Quicksilver it will be the Age of Ultron one not the Even Peters version.
Mystique isn't coming back, Fassbender hasn't signed on for another one, neither has McAvoy. The thing about Merging Universes is that the X-MEN franchise in of itself is a multiverse, full of plotholes, retcons and a nonsensical timeline it would just raise far too many questions.
Legion itself is in a parallel universe which has the promise of future films taking place in that universe that Legion and possibly Deadpool inhabit.
Marvel should recast the X-MEN and their villains they will do a great job at it, like how they did a great job at recasting Spider-Man. Deadpool should be left alone though which considering it's a spin-off(which are under control of Fox and Sony it probably will be left alone.
Also even if it were to be a universe merger, your not going to get Shipp or Sheridan anyway, due to the fact of that movie being a period piece taking place in the X-MEN Days of Future Past timeline, you will be getting the older X-MEN, not the younger ones. Unless the ending of DOFP takes place in the Marvel Cinematic Universe which it doesn't, because if that was the case then Logan would have made a mention of the Avengers, but it hasn't.
They can't merge with the 80s X-MEN because it would create a time paradox, and a giant plothole, and raise too many questions.
If you want Ryan Reynolds as Deadpool you ain't getting Shipp, or Sheridan even with Time travel because it would create a paradox and a cluster**** that would cause audiences to not like the movie in which it happens.