I know two things for sure: the reaction to X-Men: Apocalypse was mixed (I thought it had some real problems, but enjoyed it overall), and there will be another X-Men film, nonetheless.
Here's what I would hope to see from this film:
1) The team actually saving some humans. Seems like all the previous X films were so engrossed in the "we're fighting each other" trope, they never had any time at all to do some real superheroing. Yes, I realize that saving the whole world means saving humans, too, but I'm talking about some specific rescues and heroics; stopping a bank robbery or a nuclear weapons theft, hell, thwarting a mugging! Let's see them connect and interact with average joes for once.
2) New villains. I know we're going to get Sinister, but I agree it's time to shelve Magneto for a bit. Loved McKellan and Fassbinder in those roles, but too much saturation can make anything stale. Nimrod, Selene, Freedom Force, Fenris, or the Brood might work well.
3) The team acting as one; and for more than five minutes at the end of the film. One of the things I loved about the X-Men as a kid (and I'm talking the Claremont era) was the way the members worked as a team; rehearsing, conflicting, complementing, etc. We have seen precious little of this in the films, and it's high time.
4) One core group of six or seven. I think the creators of the films so far have been so concerned about putting as many X-Men in as they can, they have ended up only being able to focus on a very specific few (i.e. Magneto, Professor X, Mystique and Wolverine). Drop down the ranks a bit, and that will stop the need to have a half dozen new origins in each film. And you can concentrate on the wonderful character interaction that hallmarked the classic X-Men days.
5) MCU Crossovers. I'd LOVE to see a cameo by a MCU star like Thor or Cap, but I suppose given the Fox/Marvel split over the characters, this just isn't going to happen. Still, no harm in dreaming.