*Please go easy in the comments as this is my first article*
Without any of the four characters listed above, the main trilogy will not have been as intriguing and entertaining as it was. They all had cool powers and they all had compelling character traits that the other mutants never had. We were interested in a woman who perfectly mimic someone's look and voice, and by a mutant who can't open his eyes or he'll incinerate everything in sight, by mutants who had major destructive potential but still had yet to reach it, and by the very principal and founder of the Xavier Institute. Jean's transformation into the Phoenix could have worked, but the screen translation was completely heartless and lacking in substance.
So where does Fox continue to make money? Prequels, of course. Now that the potential for any sequels to X-Men 3 is now non-existent, we now must dwell to the past.
Wolverine was terrible, just terrible. It made its predecessor look like it ought to rank with Bryan Singer's classics and it had more plot holes than Swiss cheese. I still haven't seen First Class yet, but I hear it's very good. Sequels are being developed for both films and I still believe they can be good.
The problem is, that dreaded third film which killed off all of X-Men's good characters.
Treatments are being developed for X-Men 4 and I have very little faith in it, and it would take somebody on Christopher Nolan's level in order to make it interesting and keep the concept alive. In this lifetime, that's not going to happen.
The sequels to the prequels I believe can still work, with the right director and the right story line as we have learnt from the past how much difference one director can make. The only stories in the series that I think have much potential in them are the ones that are being made at present, which are Wolverine's, Xavier's and Magneto's and the Deadpool spin-off's potential is very limited as we barely got to know the character in the little screen time he had in Wolverine.
After the prequels are done, the dreaded R-word will have to be enforced: reboot. It's what this franchise needs after it has collided into the messed up direction it's fallen into. I admire the plots of the main trilogy, but they need better handling and direction, particularly the third film. Fox are officially the worst studio for adaptations of any sort and if any reboot is to be done, it absolutely must to go back to Marvel.