I have gone into all of the X-Men movies with cautious optimism, which has been met with mixed results.
X-1 was an opportunity to finally see the characters come to the screen but the biker costumes and "grounded" approach dialed down the comic influence.
X-2 was a great story.
X-3 introduced more fan favorite characters, most of which were crapily done and the story was terrible.
XFC was a soft reboot which breathed in new life and showed that the franchise could improve with a new director at the helm.
DoFP was a very good story and would have been the perfect way to close out the Singerverse and completely reboot the franchise with a totally new creative team. They had their "reset" ending and all the set pieces were in place.
But unfortunately, Singer just couldn't let go which led to...
X-Men Apocalypse. While that movie did have its positive points and closer to the comic source material, it was a mixed bag. Between the mediocre writing, so-so casting, poor handling of characters, shoehorning in of Mystique as a hero/team leader, unnecessary Wolverine cameo (which hamstrung the opportunity to recast the character) and diminutive and poorly designed Apocalypse, it was a huge missed opportunity.
I'll go into X-Men Dark Phoenix with cautious optimism but some concern. The Phoenix/Dark Phoenix story arc is one of the best in comic history and this seems both forced and rushed. While Singer has finally been pried from the director's chair (although no doubt lurking around) they hand it over to Kinberg who is basically just a continuation of the same old same old. He's basically Singer's producing partner. Keep in mind, while he did write a couple of decent scripts, he also wrote X-Men First Class, X-Men Apocalypse and Fantastic Four. Why Fox didn't bring in a whole new creative team and hot shot director is beyond me. Instead they hand over $150M+ film for his directorial debut? I'm guessing Fox is just risk averse or maybe Singer/Kinberg have somehow bamboozled them into thinking they are the only people who can make the franchise work. At $500M- $600M/per release the X-Men movies aren't making close to Marvel or DC dollars at the box office and yet Fox sticks with the same creative team. Before the Iron Man and Avengers movies, X-Men was Marvel's most popular superhero team. It is still a strong brand that Fox still doesn't know how to handle. Marvel can a little known team like Guardians of the Galaxy and turns it into a worldwide phenomenon. Can you imagine what someone like James Gunn could have done with the X-Men? And I don't want to get started on Fantastic Four, which should at least get a Sony/Spider-men type deal. That's why I don't like how Fox is handling their Marvel properties. However, it's completely possible that this movie will be great and Kinberg and the new cast will blow us all away.
That said, I have been giving some thought to what could have been...
As mentioned above, Days of Future Past was a perfect jumping off point, but to me X-Men Apocalypse was a case of one movie too many. If Fox hadn't allowed Singer to keep his fingerprints all over the project and inject his "vision" into the the relaunch, there could have truly been the opportunity to set up an all new X-Men cinematic franchise influenced by Cockrum/Byrne/Lee comics. Instead of immediately jumping into the Apocalypse storyline, they could have taken their time to tell a well connected new X-Men trilogy.
MOVIE #1 -- could have introduced the new team members (Cyclops, Jean, Storm and maybe Beast, Angel and Iceman (free from the shackles of past casting). They could have still had some of the other characters cameo but not as the main team. They could have also done away with the entertaining but wasteful Quicksilver bit and saved characters like Havok, who could have once again been Scott's younger brother thanks to the timeline reset.They could have also firmly reestablished Fassbender's Magneto as the villain (he could have experienced the same family tragedy at the hands of humans) and made him the leader of the brotherhood of evil mutants, which could have included the likes of a recast Mystique, Juggernaut or Blob, Pyro, Toad, Quicksilver (could have still been the actor but with an altered upbringing) and his twin sister Scarlet Witch (why not?) and maybe Psylocke. The story could have harkened back to the classics and included some amazing team vs. team fights that could be as good or better than the CA: Civil War airport battle. Also, note that without the Jackman/Wolverine cameo in X:A, the character could have been recast without impacting "Logan".
MOVIE #2 -- Now you can do X-Men: Apocalypse. With the team building out of the way, the focus of the storytelling can be on upping the ante with an even more powerful villain. I actually liked much of Apocalypse's backstory and power set, as well as the motivation to capture Xavier. Unfortunately the rushed production was poorly written and the character design was dreadful. After the events of Movie #1 with the X-Men squaring off against the brotherhood, there are mixed feelings about mutants. Most people and politicians will still fear them, but some will view the X-Men as heroes after seeing them take out Magneto. That's when a new threat will emerge. An 8' tall Apocalypse would have been truly threatening and his Horsemen could have been horrifying. By making Magneto a horseman, Conquest, they not only keep Fassbender/Magneto, but the redesign of the character and costume between movie #1 and #2 could have shown how Apocalypse corrupts the mutant in the transformation, rather than just looking like a Project Runway costume design. With Angel as a team member in Movie #1, you also get the arc of seeing Warren transformed into Archangel of Death (which could have had blue skin, red or white pupiless eyes, and razor sharp wings). Perhaps Psylocke could have been transformed into the Horseman, Famine and there could be a developing love story between her and Archangel, which eventually breaks the two of Apocalypse's control. Juggernaut would have been the perfect War Horseman. In the end it would take the combined strength of the X-Men, Brotherhood members Quicksilver and Scarlet Witch, defecting Horsemen Archangel, Psylocke and Magneto to match Apocalypse. (Not sure if Juggernaut defects as well or stays bad). Pulling in more and more power Apocalypse grows to 100 feet tall and as the fight begins shifting in his favor and it becomes clear that he's going to defeat them. In this telling the Phoenix force has been imprisoned in the M'kraan Crystal which is embedded in a ceremonial statue used in Apocalypse's resurrection ceremony and he taps into its power. Phoenix is from Egyptian mythology, so it can play nicely into the storyline. In certain respects Phoenix is like a genie trapped in Aladdin's lamp. During the battle with the X-Men the crystal could have cracked allowing the force to escape and seek out an ideal "host vessel". That would be when Jean Grey's power suddenly goes off the charts, the Phoenix force is first manifested, and Apocalypse if engulfed in flames. The X-Men's victory celebration will be short-lived when they realize that Jean has fallen into a coma and is near death.
MOVIE #3 -- Now you are set up for the Dark Phoenix arc. Jean is not dead. When she comes out of the coma she seems completely back to normal. As they all get back to their lives for a while it will seem like everything is starting to go their way. There are no villains or criminals to fight, politicians aren't railing against the mutant threat and hate groups are not calling for their heads. They are getting back to school life and being teens and young adults. However thousands of light years away a Shi'ar deep space science vessel detects the Phoenix force energy signature--an ancient threat long thought gone from the galaxy. They immediately contact the homeworld. There was a reason the Phoenix force was imprisoned in the crystal. Thousands of years ago their ancestors had managed to defeat the immensely powerful and sentient force by constructing the purest crystal in the universe to contain it. However the crystal had been stolen more than two thousand years ago, and was lost during a massive space battle to retrieve it (think Starfleet vs. the Romulan Empire). The crystal eventually ends up on Earth and in the hands of Apocalypse in ancient Egypt. That would then be the backstory. Shi'ar emissaries come to Earth, and discover that the force is residing in Jean, which they report back to the Shi'ar High Council and Princess Lilandria, who are on board the flagship positioned behind Earth's moon. They know that the only way to release the Phoenix force is to destroy her host body and re-imprison the entity while it is still weak, the same tactic used millennia ago. Lilandra chooses to meet directly with Xavier to explain what must be done. Xavier will have nothing to do with it and vows to protect Jean. Meanwhile as the Phoenix force ha been growing stronger Jean's personality is growing weaker and she has been slowly losing control but keeping it from her friends and teammates. Lilandra issues an ultimatum, saying that if Xavier refuses to turn Jean over they will have no alternative but take her by force. She says that the technologically primitive planet earth wouldn't stand a chance against the Shi'ar fleet and they will reduce the planet to ashes if necessary to stop the reemergence of the Phoenix. She gives them 24 hours to turn her over. That's when Xavier reads her mind and learns about the ancient but highly respected Shi'ar custom of honor trial by combat. This sets up the X-Men vs. the Champions of the Shi'ar Empire. To keep Fassbender in the franchise along with characters like Quicksilver and Scarlet Witch, Xavier can go to Magneto and convince him that this is threat to the entire planet. This can then set up the battle on the Moon with the X-Men vs. the Imperial Guard led by Gladiator,. When the X-Men are on the verge of defeat Jean goes full on Dark Phoenix, which is far more terrifying than Magneto and the Brotherhood from Movie #1 and Apocalypse from Movie #2. She immediately incinerates the three Shi'ar starships (the flagship and two flanking ships) which had a combined crew of more than 2,000. Lilandra and the high council members survive, along with about 25 crew members, because their Imperial shuttle had been on the ground. Rising above everyone Phoenix proclaims that it is finally free and in control of the host vessel. All trace of Jean appears to be gone. This will unite the X-Men, Brotherhood members and Imperial Guard. But it will take Jean and Scott's love to defeat it. That's one reason that casting is so critical, because I didn't see much chemistry between Turner and Sheridan which would need to be there for the story to work. In the end Jean will sacrifice herself (no resurrections) and the Phoenix force, which can never be destoyed, will be recontained in the crystal because it was the only way (Xavier was wrong on this one). That would cap off the trilogy with both a victory and a sad ending, but open the door to future stories.
That's my quick take.