If you had never read a comic book about the X-men, X-men: Dark Phoenix was a good capstone movie to the franchise. It had action, heart, good special effects, aliens, and wrapped up stories that had been in all the other X-men movies.
The problem is that X-men Dark Phoenix is based on one of the most well-known comic book stories of all time.
Much like the last Star Wars movie that emphasized all the wrong parts of Star Wars, X-men: Dark Phoenix took all the pieces of the comic story but wove them together in a way that was NEVER going to please any X-men fan.
1. Death of Jean's parents. Do we have to start every movie with a death.
2. D'Bari as alien menance. The D'Bari are at the center of the Dark Phoenix's evil, victims of it's twisted power, not Hellfire Club inspired, super-fast, tree monsters.
3. If we need aliens, where were the Shi'ar?
4. Jean Grey is a vibrant super-model, not a sheltered wall-flower.
5. Where was Mastermind?
6. Jean does kill, but not Mystique.
7. Nightcrawler and Magento are German. No accents in the movie.
8. Beast is blue all the time. He's not the Hulk.
9. Who are the no-namer mutants who help Magneto? We have an entire catalog of mutants. Why are we making up some new characters?
10. The last Jean Grey movie was a disaster. Why did we borrow her wardrobe? Long coats again.
11. The Phoenix is a giant, cosmic bird. Most of the movie it's a fire blob.
Maybe Disney will learn from failed mistakes and give us a proper Phoenix trilogy of movies.
1. X-men: Shi'ar - Xavier saves Lilandra, Jean is consumed by the Phoenix.
2. X-men: Hellfire Club - this is what happens when mutants go bad. Jean is manipulated by Mastermind into becoming an evil version of herself and this twists the Phoenix into Dark Phoenix.
3. X-men: Phoenix Song - Bring the first two movies together, ends with Jean sacrificing herself to save the universe.
The goal here is that America falls in love with Jean Grey in movie #1. She's America's sweetheart. She's everywhere on everything. In movie #2, every guy wants a bad girl. In movie #3, Jean returns to the woman we know and save her friends.