I sure hope it's an actual spin off and features all new characters but I've said before I have a feeling it would be a continuation post apocalypse. My idea is that after the events of apocalypse Nightcrawler will leave and go do his own thing. Beast will also go off. Prof X plus Cyclops, Jean, Storm, Angel and Jubilee can recruit Cannonball and Mirage. Selene can be the main big bad. She becomes obsessed with reforming the now dead Hellfire Club. So she can be queen and be worshipped as she has her whole existence. This is a way to bring back Magneto as she tracks him down to get information. He doesn't side with her and goes and help Xavier with the new mutants and to take down Selene and her recently reformed Hellfire Club. I have this idea that Selene could be after Mirage which is when her grandfather is killed and Prof X helps her and takes her in.
We can see Mirage trying to learn her powers and focus them but she also makes mistakes like revealing some dark fears of X-Men. Could be a great scene like in Avengers with Scarlet Witch. Have mirage project Storms worst fear. Or Magneto's even.