What can we expect from this film? Well, in the original storyline, Mystique's Brotherhood of Evil Mutants assassinates a senator, which results in a dystopian future in which mutants have been hunted down by Sentinels and killed or imprisoned. The now-adult Kitty Pryde sends a message to her past self, and the X-Men are able to prevent the assassination, avoiding that future.
So, first change: Magneto. He wasn't a part of Mystique's Brotherhood (the films basically have Mystique be Magneto's top minion, which the comics to my knowledge have never done), but they're not going to waste a chance to use Michael Fassbender. Perhaps he'll have found out the politician ultimately responsible for the "shoot" order at the end of First Class was JFK?
The temptation to have "the future" be 2014 will be very strong. We'll see a distorted version of today's world portrayed as a dystopia - mutants being rounded up and kept in camps. Think the early parts of District 9 for tone. And for the sentinels: I'm sorry, but we are not going to see giant humanoid robots with funny hats on film. Even if it weren't too silly, it's too similar to Transformers 4, which will be hitting screens less than a month before X-Men: Days of Future Past. Instead, I wonder if they'll resemble unmanned aerial vehicles (UAVs), which is basically what they are. Maybe have them be called "sentinel drones".
The time travel in the original storyline is a mind transference, which will be too understated for a big-budget action movie. Although First Class is irreconcilable in many details with the original trilogy, they still kept a kind of loose continuity for the cast. This means Kitty Pryde is not going appear as a youngster in the 1960s era, and therefore probably won't appear as an adult in the 2014 segment, either. It might be any of our remaining cast of X-Men, but my money is on Rachel Summers coming - body and all - from the future. This makes it a personal story for Rachel's father, Alex Summers (Lucas Till). Yes. You know it makes sense.
Making Alex be Rachel's father is completely against the "canon", of course. But you know, so what? The film series has never attempted to recapitulate the original stories (the first one dealt neither with an attack on Cape Citadel nor the rescue of a previous set of X-Men from a living island), and it's often made massive changes to characters - look at Rogue, for example. And I do not think it's necessarily the worse for that, it's just different. X3 is rubbish because of its structural faults as a film, not because it failed to followed Claremont and Whedon closely enough.