A Geeky Viewpoint

It sounds like both Quicksilver and the Scarlett Witch are planned for X-Men: Apocalypse. 

When asked about Quicksilver Simon Kinberg said "It wasn't something we anticipated, because we didn't know they were using him in The Avengers movie. It got publicized weeks after we made the decision that we had done it, and then after that even, I think it was a little while before the controversy started in terms of them using him too."

Simon Kinberg followed up with "We talked about bringing Scarlet With into the mix for this film, and there was actually a little scene we shot that we ended up cutting from the movie that eluded to her. But it was a sort of interstitial scene that didn't push the movie forward, and so it ended up being cut, like lots of bits and pieces in a movie that's this complicated and with this many characters to follow. But she would absolutely be back for the next X-Men movie."