I want to be excited about this...but I'm definately cautious about it. I've liked Bryan Singer's X-men material so far. Still, although there's a significant time gap between First Class and X1 that allows for a prequel-sequel to exist I'm not to sure about it.
Heres' a potential plot. Now that mutants are known armies around the world want their covert mutant faction. Mr. Sinister could be a great villain this time around, let Mags sit this one out as the main antagonist. Plus, Sinister is one of those unaging mutants so he can appear whenever without screwing up the timeline. If Havok must be in this timeline, then turn him into Cyke's dad. Adding Mr. Sinister to this equation can go with comic cannon as Sinister has this unhealthy obsession with the Summer's family. To fit with the movies, and probably add Mags, now that mutants are around, Sinister is trying to make them enhanced mutants he loves. Magneto is partially swayed to this as the he deems humans inferior. However, after seeing the heinous extremes Sinister will go to achieve his chimeras he is reminded of Nazi experimentation and bails out, but Prof X and team must go in and clean up Sinny's mess. Fox could even cameo Wolverine again with a parallel of this enhanced mutants and genetic crossings with Weapon X program. In fact, you could return Moira to her usual role by having her head a lab that studies mutants due to her experience with them in First Class, aided by Prof X.
Seem good?