This movie's rumors caught me off guard so I wanted to sit and ponder them awhile. These rumors were reported on Bleeding Cool, and on various other outlets.
Not only will we be dealing with period assassinations, we might be seeing the future too. Rumors and comments of Cable, New Mutants spin off, and more. The current running rumors are that it's going to potentially have Cable, use an adapted Days of Future Past storyline or just the title for the time travel element, and be centered around the Kennedy assassination.
We also need to consider that the serial self contained multi-title universe feature building has been proven to be a moderate to decent success with fans, TV viewers, and movie goers at large in succession. So this opens an interesting door for Fox. Granted it seems the entire purpose of a future story (*Cough* young justice *cough*) is to change the future. That still means these various mutants shown could prove potential for a spin-off same universe feature.
Could prove for a decent testing ground for any character to make a cameo and be able to gauge audience reception. This is of course refering only to the future landscape or side.
The past period is limited moderately so from previous films. Let's see what we've got to work with if they keep the details.
The timing seems difficult since we're still in the 60s in comparison to other recruits. The X3 Jean Grey flash back would have to be checked for her time period(check for newest model car or appliance).
The three mile island incident sets a date on by when Xavier needs to go bald. So that gives us a year there for Cyclops too.
Emma Frost(getting old? Someone Shatters her?) the telepath gaining a new body from young Emma Silverfox after the 3 Mile Island incident(Which makes Charles learn about leaping bodies as shown in X3 and his lesson plan on Coma patients).
The future inbetween flashbacks period doors are also wide open for any background character teaser to gauge reactions. So we could have Cannonball's farmhouse, Dani Moonstar from her reservation, X-23 and her creation(leaves more room for when she tells her story to megan in an innocence lost feature). The doors are open for anyone, maybe even archangel and Apocalypse. Really the windows are open too for anyone. Even Dazzler and Pixie. If they talk to Ellen Page back, could they get her as an older Kitty Pryde, or will they continue Kitty Pryde's running gag of a different actress every film? What about Colossus? Hugh Jackman I could see for the future story line too, so they can cash in on his character and try to gain more audience appeal. Lots of potential on this end, just to name a few. In an even scarier way, the rumors of Cable in the past set up the potential for Cable and Deadpool feature at some point if they manage to actually do a Deadpool feature that fixes his issues back to who he is with the costume included. He's the Spider-man of the X-Men. Just doesn't work without the suit and wisecracks. Thankfully this just means you only need Ryan Reynolds voice which frees up the filming schedule if that feature were to ever exist.
So c'mon Fox. I dare you to take X-Men to Avengers heights while maintaining a cohesive X-Universe. Make your franchise the star wars of comic features that focus on the key points of the mutant war. Discovery(XFC), Magneto killing a major politician, Birth of Weapon X(XMO:W), Mystique becomes a senator, Destruction of Weapon X(X2), the red haired lady incident and cure injections(X3). That leaves them open for another feature before the original X-Men feature after this one before they need to dive into an official X4, of course this split could also be used to bypass the serial titles and just append an immediate continuation of both eras until they formally meet as a variation on the multi title system.
Though that'd be a bit pecular and forceful.
So churns the rumor mill, and decisions they need to make.
Keep your fingers crossed for whoever you want to see.