While some of the original "X-Men" trilogy cast members were showing up in "X-Men: Days of Future Past," up until now the plan seemed to be that the next film in the series would return back to the "X-Men: First Class" cast only.
Then came hints that Hugh Jackman's Wolverine would also be back, and now screenwriter/producer Simon Kinberg seems to indicate that at least a few more may also be back. Kinberg tells The Daily Beast: "[Apocalypse] will focus primarily on the First Class cast, but it will certainly have some of the original cast involved too."
How this will work isn't quite clear as it has previously been indicated the next film will be set in the 1980s. It's a time when various previous trilogy characters could appear, but they would be younger versions (many of them kids or teenagers) and so would be recast.