The only thing they are really bound to Is fact Wolverine In 2023 will be at mansion as Teacher and so will Kitty & Colossus-possibly In romantic relationship like comics,Beast-who will accept his mutation like version seen In Last Stand,Storm,Jean Grey,and Cyclops.Iceman and Rogue are also at mansion In romantic relationship like one they had In trilogy.Xavier Is running school In hoverchair-like 1990's animated and comic version.
Rogue,Iceman,Colossus,and Kitty won't be 1980's and 1990's set films.But beyond that anything goes.
You already have Xavier,Magneto,Mystique,Wolverine,Beast,Apocalypse,Cyclops,Jean,Storm,and Quicksilver set for Apocalypse.Plus Nightcrawler and Gambit while not
100 percent confirmed are good possibiltys to be In Apocalypse.Then there are others like Scarlet Witch and Deadpool who could show up In film.They have tons of characters they can use without iceman,Kitty,Rogue,Colossus.
Kinberg's comments about characters being born earlier may be about those who showed up briefly In past films like Psylocke.
DOFP was already somewhat of a reboot.They want to make the FC contunity the one series follows.But the want to try to silence fans of Trilogy by saying there Is still "connection" to those films and new films.Thus much of original cast In ending of DOFP.And they have opotion of using cast again If they choose even though contunity of trilogy Is out the door.