Days of the future past will take place between two timelines. The main timeline being current First Class timeline where Xavier and Magneto have just parted their ways and then there is a post apocalyptic timeline which is likely to be ruled by Sentinels. Here is how the duel trilogy’s idea starts.
1. What happened after X-Men 3: To be honest before I joined this site I liked X-Men 3 but public opinion forced me to change my mind. By the time X-Men 3 ends Cyclops is dead, Jean is dead, Magneto and Mystique have lost their powers, Logan returns back to Canada and Charles Xavier’s consciousness is transferred in another man’s body after his death, this is quite a shitty future and not what the Xavier of X-Men First Class dreamt of. With no one to lead X-Men bad forces come out in open, let it be Apocalypse or Stryker’s Sentinel project. After things get out of hands and with a total destruction the need to warn someone in the past is the thing post X-Men 3 remaining mutants need to do.
2. Xavier’s Death: When Jean disintegrates Xavier, he transfers his sub consciousness in mind of another patient. If you try to look in where his consciousness in between of the film then there are three possibilities. A) At the island watching the battle or helping somehow. B) Accidentally having a glimpse in the horrible future and then C) going back to meet a young Xavier to warn him, or may be when Jean was disintegrating him gave a glimpse of the future that made him travel in the past. If DOFP goes through this route it can save a lot of trouble of setting the story.
3. Moira: In X-Men first class timeline Moira works as a spy in a society which is still not open with females having a lot of freedom in working alongside men, by the end of the movie she even gets a demotion. What happens to Moira’s career and struggle is finally seen in end credits of X-Men 3 where she is now working as a doctor or something like that. Days of the future past can give us more glimpses in the life of Moira.
4. Jean Gray and Cyclops: They are yet to be introduced current timeline and Jean may serve as a reason why Magneto and Charles reunited in the first place repeating the scene from X-men 3 as an easter egg.
5. William Stryker and Senator Kelly: Stryker had a cameo in First Class. He clearly comes a long way in his career in X-Men original trilogy. He has a mutant son born in First Class timeline and wreaks havoc in first trilogy’s timeline at a point. We don’t see him dying by the end of X-Men 2 but whatever insult and humiliation he goes through in that movie mast have pushed him towards unleashing the sentinel project.
In original Trilogy universe Senator Kelly is dead, another push that in DOFP storyline started the Sentinel project in the first place. X-Men first class may change this a bit with linking it to some historical event but that doesn’t mean there are no chances to meet a Robert Kelly who is on a start of his career.
6. What happened to Wolverine: Another question whether we’ll see Hugh Jackman as Wolverine in this sequel, if he joins it can show what happened after he left X-Men and returned to Canada. Is he alive in that timeline or among other casualties or maybe we get to see a Logan returning from War in First Class’s timeline getting another cameo.
Days of Future Past timeline is a spot which can give Fox a lot of advantage and opportunities to fix a lot of errors and issues and a good filmmaker will make use of it. You don’t even need the old cast to set into the alternate timeline, all you need is some posters or some gravestones or may be a mutant breaking into Xavier’s school and running a past trilogy’s video feed having some flashback scenes and go ahead with story.