By the time Marvel gets X-Men back, Ian Mckellan's real children will be old enough to have had their own children, who are now old enough to play Ian Mckellan in a documentary about what he did today.
How are they getting Quicksilver when he was in X-Men? Did Marvel and Fox come to some arrangement to allow the crossover or will we be seeing a non-Bacon Burger Quicksilver this time around?
There was a licensing thing where since the Maximoff twins were originally Avengers long before they became mutants and involved with the X-Men, that Marvel can still use them as long as they don't refer to them as mutants, and never mention that they are fathered by Magneto, another mutant, as Fox owns all rights to anything mutant in the MCU.
Fox on the other hand can also use them, but can never mention anything regarding The Avengers.
Marvel and Fox are not on good terms at the moment, and there will likely NEVER be a crossover between the two camps movies.
Beef supposedly started over use of Galactus and Silver Surfer, and now Marvel has supposedly retaliated by not producing (or cutting back on) any toys or comics related to the two properties that Fox is in control of. X-Men and Fantastic 4.