Chris Claremont is a comic book writer who has been handling the X-men series from 1975-1991 which is a very long time especially in the comic book world. Claremont spoke at a New York Comic Con panel he discusses exactly what he would do.
Since it was just a quick discussion, these ideas shouldn't be taken too harshly or critiqued too hard as writing for a film takes a long time. With that being said, what the comic book writer suggested would've made for a very interesting film considering the last two films didn't do well during or after their release.
Claremont says one of the things he would change is to introduce Sophie Turner as Rachel Summers during the ending moments of X-Men: Days of Future Past (the scene when Wolverine meets up with Cyclops & Jean Grey). In the following films such as X-Men: Apocalypse, Claremont would have Jean having multiple nightmares about the Phoenix power/persona she adopts. This would happen as a result of seeing the future through Wolverine's mind. Instead of possessing Jean, the Phoenix would take over Rachel and during the "present" day the original Charles Xavier would use the newer one as his "mental image." He would again have Professor X be defeated by Apocalypse, but he would have tried to take Jean's body instead of Xavier.
Finally, Claremont would have Rachel defeat the villain by unleashing the power of the Phoenix into the cosmos. Similar to Claremont's comic book run, he believed the alien force should consume a star that would set the pieces for the Shi'ar to come to Earth. The Shi'ar are an alien race from the Marvel comics that appeared in the X-Men comic book series. Assuming this world tied into the Marvel Cinematic Universe (MCU), this would tie perfectly with the cosmos part of the Marvel canon (Galactus, Eternals, Silver Surfer, etc).
At this point in time, it's unclear if anything could've been done to make the films have a better reception. The stories were so far down their route that some storylines may have been too far from saving. It's also hard to translate comic book storylines into the confines of a live-action film due to the restrictions of film length and story pacing, as well as physical and financial limitations.
The full interview can be heard over at AIPT where the writer goes way more in-depth. X-men fans would love to hear more and explore the universe considering how deep it goes. Hopefully, they don't have to wait too much longer before they join the MCU canon while it's still on top of the box office.
Source: AIPT