I have this feeling that the mutants that will appear in the dystopian future (assuming that is what occurs) will not be the ones we've been speculating about (Xavier, Magneto, Wolverine, Storm, etc.), but the New Mutants. There was news about Fox wanting to do a New Mutants movie a few months ago, so rather than doing a whole movie on them, they'll show up in DOFP (kind of like the way much of the Magneto script wound up in First Class). The upside for Fox would be that they'd hire younger, cheaper actors as opposed to bringing back actors from the OT who won't come cheap.
I hope I'm wrong