The x-franchise has been handled relatively well by fox, but didn't realize what sort of gold mine they had until the avengers run (which, for all it's fault, is one of the greatest moments in cinema history)
What characters they developed, they developed well. Professor X, Magneto, Wolverine, Beast and heck even Mystique have personality and heft. I joke that the x-movies are the "Charles and Erik show", but that's not a dis on the characters as all actors that have taken those roles have done it well.
There is also never been an x-movie that didn't start strong. I will always argue that the first few minutes of x1 are one of the reasons superhero genre movies exploded, and even stinkers such as wolverine origins, had a great opening scenes and opening credits.
However, what they missed is the development of multiple characters.
I will use x-men 3 as my major complaints to point how they could use better.
For starters pick one plot and stick with it.
Wolverine, Beast, Professor X and Magneto were spot on and don't really need to be touched. It was the plot that let them down.
also the Pyro/Iceman rivalry was surprisingly well handled for what it was.
1. Keep Jean on Ice and do the mutant cure story arc. Have Cyclops show why he's leader as he comes to terms without his significant other. Have him turn inwards and start the transition from dork to magnificent genius.
hell, we never got the lenses comes off with Scott and that would have been awesome. :)
2. Fill in the holes with the next generation. You have a team consisting of Rogue, Iceman, Colossus and Kitty Pryde. That's a team that should be coming of age as they fill in the holes against the brotherhood assault. Hell, use a character or two as background ops (such as that kid who changes channels with his mind in x2. Show him to be their "decker" and use him for exposition). By developing the next generation, you won't need to do reboots when your original actors get old.
3. And by all means have Rogue do something. You have this great build up over 2 movies that goes nowhere. If you want to reduce the Ms. Marvel shenanigans, have Rogue hold onto Juggernaut to get her super strength and invulnerability so she can keep up with the three (and explains also why she becomes more sassy). Hell have the fight so that it was only by the teamwork of Colossus, Kitty Pryde and Rogue's power drain that they managed to stop the Juggernaut. Plus you can now draw in the tweens with 4 strong characters and have an option if you cannot rehire the cast for the next 3 movies.
3. less of a mutant conga line. I don't mind Warren, but he could easily be written out without any real change. I like their version of Calypso and honestly she seemed like a bigger fight in the movie then in the comics.
If only storm was up to the task. :p
but the others, darm if I knew whom any of them were even after showing the credits. I guess you could pull a mini vid on some of them in the extras, but honestly, a strike team of Magneto, Mystique, Juggernaut, Sabertooth, Pyro, Toad, and Calypso is good enough to require a solid line of x-men. Maybe Emma Frost if you want to use her in x-5 against Jean.
If you want more mutants, introduce them as protesters in the background to draw on in later movies.
Then as they were fighting and the fight is won, then wake up the Phoenix with a to be continued.
you now have the groundwork for x4: phoenix rising. Where the brotherhood and the x-men have to team up to take down the greatest mutant threat.
which ends with either days of future past or awakening of apocalypse for X5.
The other issue I have is the side-lining of minorities and female characters.
The x-universe is often an analogy of minorities, but you wouldn't believe it from the movies.
Hell, until relatively recently, the prominent marvel female was Storm: a black female mutant who was usually in a leadership role, and for good reason.
However, they chose a actor that didn't quite get the character: Namely a regal drama queen with a deadpan sense of humour. As a result, the character, despite being advertised hard, never was able to fill those shoes and got sidelined hard as the series progressed.
Another example is in first class.
When they did the split of the teams, the minorities went into the brother hood, and the white characters stayed as x-men.
Worse, one black character, Darwin, was killed specifically by Gideon, despite having a power that should have allowed him to adapt to it.
Even if unintentional, that has stuck up my craw to this day.
Asian characters have faired somewhat better, Lady Deathstrike, despite being a flunky, had a few great fights, and the Wolverine really did showcase Japan well.
But still, Jubilee was literally put on a bus in x-2 and that bugged me as well. You don't give Jubilee a non-speaking role. That completely defeats the purpose of the character.
here's hoping that Psylocke is a break out character in this regard.
Ultimately the x-fox movies are like being at a smorgasbord, and only eating the salad.
Its okay to try the other stuff. Marvel Studios would kill to have this franchise back as it literally drove the company for decade and has a ton of amazing characters of all Race, Sex, and other personalities quirks that could drive stories.
May they one day have the guts to do a Spiral movie (One part reality tv, one part ex machina, one part human centipede, with a healthy dose of Hellboy mixed in. :D)