To put it kindly, the Fox X-films as a whole was a mixed bag. They've produced some of the best, like Logan, and some of the worst, like X-Men Origins: Wolverine, but ultimately it wasn't until the end of their run that they began to truly play with the genre but it was too late. By then the topsy-turvy nature of Fox's relationship with the films and audiences had taken their toll.
For comic readers, in particular, the Fox films rarely captured the depth and vastness of the source material by playing it safe and going with easy to understand dynamics.
It wasn't necessary for Fox to adapt specific stories - although Logan may beg to differ - but there were pivotal moments of the X-titles that were ignored to keep the plots simple. Seeing how the MCU was able to keep the stories fairly simple while not dismissing the biggest moments of their characters, it became all the more glaring when Fox missed the mark.
These omissions, alterations or just flat out fundamental aspects of the X-Men should not have been ignored by Fox and the screenwriters...
10. Kitty Pryde's Introduction
Kitty Pryde was technically in the original X-Films, but it wasn't until The Last Stand with Ellen Page in the role that she was given anything to really do. She was there to be one of the sides in a love triangle between Rogue and Bobby Drake with very little else to it. It was underwhelming.
Without going into the entire history of Kitty Pryde and her myriad of codenames, her introduction to the X-Men comics was a huge event in the war between the Hellfire Club and the X-Men.
X-Men #131 as a whole is Kitty Pryde being recruited while both groups vie for her to join their side. More than being the last leg before the Dark Phoenix Saga, it was a story filled with the basics of who the X-Men are and why they are not just another superhero group. It was also a pivotal moment bringing in a young Jewish girl to act as an audience proxy and fully incorporate what being a 'gifted youngster' would be.
She's become a staple character in the comics, and something like this introduction is needed in future films.