If you haven't seen X-Men: Days of Future Past in theaters yet, there will be plenty of spoilers below, so read on at your own risk.
While the end credits scene of X-Men: Days of Future Past helps set up X-Men: Apocalypse, the story helps broaden the X-Men universe in general, merging several cast members from the original X-Men trilogy with the 2011 prequel X-Men: First Class. Director Bryan Singer revealed in January that X-Men: Apocalypse will be more of a sequel to X-Men: First Class, featuring the younger version of the mutants set in the 1980s.
"It will focus primarily on the First Class cast, but it will certainly have some of the original cast involved, too."
It seems things may have changed with the story between now and December, when Simon Kinberg said the sequel would "not necessarily" follow the original X-Men. Here's what he had to say back in December.
"[Apocalypse] really follows the First Class cast. Meaning it is a sequel that will feature (James) McAvoy, (Michael) Fassbender, Jennifer Lawrence, Nic Hoult and that cast. It is not a sequel that would necessarily feature Ian (McKellen), Patrick (Stewart), Halle (Berry) and the original cast. It would take place roughly between the past of Days of Future Past and when X-Men first started."
The ending of X-Men: Days of Future Past brings up several questions that will have to be answered in X-Men: Apocalypse, since it creates a new timeline that will seemingly alter the destiny of Mystique (Jennifer Lawrence), while bringing Jean Grey (Famke Janssen) back from the dead. Simon Kinberg revealed that this new timeline was created to make amends for X-Men: The Last Stand.
"The original outline, the first thing anyone read - the studio, the producers, anyone - it was something me and Matthew Vaughn worked on together. In that original outline, the characters that come back at the end of this movie came back. For me, the fun of this movie from when I said, 'We should do Days of Future Past,' was literally the scene of changing the future and Jean is going to come back and Jean and Wolverine are going to have a reunion. Mainly because I carry such guilt over X-Men: The Last Stand. The way we killed Jean in X3 haunts me because I love the Dark Phoenix saga so much."
The ending of X-Men: Days of Future Past also showed William Stryker (Josh Helman) recovering Wolverine's body, although it was revealed that Stryker was actually Mystique in disguise. Simon Kinberg teased that Mystique's reveal will certainly be addressed in X-Men: Apocalypse.
"We really wanted to do something subtle with Stryker in this movie. We wanted it to be the beginning of the origin of him. He's in the shadows most of this film. In some ways, Stryker was included in order to trigger something for Wolverine. How would it impact Wolverine, going back in time and seeing this guy who is going to manipulate him in the future. That was just interesting. Stryker's been interesting in the books and the Brian Cox version was fantastic. But the last moment in the movie with the Mystique reveal ... there's for sure more to that. As we follow the characters in to X-Men: Apocalypse, we have to address that and make it a real thing."
"Gambit is still in-motion and being figured out. Channing made it known that it was a character that he loved and would love to play, and all the people who work on the X-Men movies are huge fans of his, so the notion of him playing it is exciting. I'm more fascinated by anti-heroes, and Gambit is one of those. I don't know why he wasn't explored in the original X-Men movies. Maybe the reason why was because they wanted to focus on Rogue/Bobby or the platonic Rogue/Wolverine relationship, and maybe there were too many similarities between Wolverine and Gambit, so in order to make it a Wolverine-centric franchise they had to cut him loose."