See, I don't mind that Apocalypse had the younger cast, but given the reset that happened in DOFP, I just wish Fox had embraced the more comic-booky nature of X-Men and headed in that direction.
"The past: a new and uncertain world..."
The 80s setting was great-- give Fassbender white/silver hair, have Xavier already bald at the start of the film, keep Hoult in Beast makeup the entire time, etc. Mystique could have worn a white ensemble similar to her comics to significantly reduced her time in the makeup chair.
Jackman could have stayed as a through-line between the franchises & popped up with Mystique too. Sentinels were part of the canon--the world governments could have restarted the Sentinel Program to take on Apocalypse too. When Magneto left at the end of DOFP, he should have gone to start Genosha/Avalon and in the 10 years that have passed, he & Quicksilver should have come to terms with their father-son relationship so Quicksilver would be by his side, trying to prove himself. Heck, bring back Schreiber as Sabretooth as a Genosha/Avalon resident w/ the actor from DOFP playing Toad.
Stakes could have been raised w/ different Horsemen choices too--Magneto, Xavier, Mystique, & Wolverine (give Logan his metal claws that way). No need to waste Psylocke & Angel in that story.