I don't think the X-films could last for over 6 films, i.e. the cast/contracts/and story development would've went dry...
I think the X-Films could've went like this:
X1

same story) Intro. of the X-Men and Brotherhood, more focus on hate towards mutants,Henry Gyrich is alive
X2

same story),Mystique recruits Avalanche while searching for Magneto, Stryker tells Logan a little bit more about Team X(mentions Sabrethooth, Deathstrike, etc), Stryker has more mind control mutants/team-battle fight scene
X3):Cyclops a more stern/harsh leader(in the beginning),Phoenix sub-plot/Jean's origin, intro Beast(but not a larger role), Rogue has gained flight/strength,Genosha Island/enslaving of mutants,Sentinels,FoH(members are Emma Frost, Sebastian Shaw, Trask, Victor Creed),Emma/Shaw want to recruit Jean for economic/power reasons
X4: Cure plot, Dark Phoenix, the BoM are back(Psylocke/Juggernaut/Mutiple Man/Avalanche/Pyro), intro. of Morlocks, Leech is a Morlock and they team up with Magneto to get Leech back, WW Jr./Angel are introduced, Henry Guyrich/WW Jr. help fund the cure, Beast has a larger supporting role, when Magneto learns of Jean's power who uses Psylocke to try and coarse her,Mystique gets cured,the war is in D.C., Prof. X goes with the X-Men and battles Jean, dies, but before he does he manages to restore a few psychic barriers, banishing Dark Phoenix, the movie ends w/Xavier's Funeral, a new school year beginning, the team standing together as Cyclop declares its up to them to keep Xaviers dream alive, after the credits is the original ending scene in TLS