I think they should put Apocalypse in the next movie. I mean magneto is on good terms with the X-Men at the end of the movie, Apocalypse is one of their biggest bad guys and he does have a habit of coming back to life more than most people, he is a master mind with tons of plans hence him coming back when it seems like he's gone, Nathaniel Essex was made into Sinister because of Apocalypse and the two have worked together multiple times in the comics. While this Apocalypse was good some people including me have found him to be a little lacking and he switches bodies in the movie which would work well if they wanted to change him a little. They could call it X-Men End of Days or something where Sinister could have a small rivalry with Magneto, Wolverine could be one Apocalypse's Horsemen of War or Death cause in the comics he's often been brainwashed or controlled by someone and we've yet to see that, and they could tie in Cable so he could later be put into the next Deadpool movie. Sinister could also be the one who cloned Wolverine and make X-23, cause it's said that the next Wolverine movie takes place in the future of days of future's past so it could tie her in the 3rd Wolverine.