Jean Grey, Mystique, Storm, Moira and Psylocke

Forget about replacing the characters, what the X-Men cinematic universe really needs is a new film crew.

Namely, Bryan Singer, Simon Kinberg, John Ottman, Newton Thomas Sigel, etc. need to sit out the next one (with their only possible involvement being in some kind of ceremonial "executive producer" role).

Don't get me wrong, I loved X-Men DOFP and I thought X-Men: Apocalypse was decent and didn't deserve the hate it gets, but the "main" X-Men movies are getting stale and tired at this point and being outshined by spinoff movies like Deadpool and Logan. It was important to get Singer & Ottman back for DOFP as they had to tie the new cast into the original trilogy, but they weren't needed for Apocalypse and it showed.

Matthew Vaughn's vision for X-Men: First Class really breathed new life into the franchise. That needs to be what happens with X-Men 7. It needs to be presented in a way that is fresh and bold compared to the previous six movies. We don't need Simon Kinberg doing "Dark Phoenix" again.

I have no problem bringing back some of the old cast/characters (Jean Grey, Mystique, Storm, Moira, Psylocke being the ones you mentioned) AS LONG AS the film's execution is innovation and is done in a way to be the start a new, third trilogy of X-Men films.

To do that, they need filmmakers who will look the X-Men cinematic universe with new eyes.