Wow... you guys make some really good points. I was wrong saying your conclusion was baseless def, but your assumption that 'returning cast size makes it a sequel' doesn't work, or else, again, Avengers - with quintuple the returning cast compared with the rest of the related films that would not have been there without IM1. Same with shooting schedules, if they shoot the dark future first (which they should, to avoid Halle's baby bump), then yeah, the people from the timeline that needs to be changed would be on set ASAP.
Singer looks at the team up aspect like the MCU films:
http://screenrant.com/x-men-days-of-future-past-movie/ said:
“I think there’s a strong desire to broaden out the universe. I mean, the X-Men universe is every bit on its own as big as the Marvel universe and I think it’s time to reach out and explore it and perhaps even bring some connectivity between the films,as Marvel’s done so well. You may see some of that, I don’t know (laughs).”
The majority of the film is set before X1. This makes perfect sense because DOFP began as a sequel to X:FC, and there's no sign that that's changed.
http://screenrant.com/x-men-days-future-past-cast-plot/ said:
The majority of it…or that part of it, takes place in the early 70’s, 1973. So Nixon is a character, and there will be different styles and automobiles. And there will be some futuristic, as well as retro, technology which you haven’t seen. There are science fiction aspects of the movie, and then there’s some retro science fiction that the X-Men films haven’t had quite yet. So we’re exploring that, which will be fun. And there’s an attitude [difference]. We discover our characters, particularly the younger characters, at a different place in their lives. Every character you discover in this movie is in a completely different place than you’ve ever seen, emotionally, and it’s about them moving through that. Hilarity ensues!
So not only are they sticking with the DOFP storyline, where people go from the future to the past, to fix things but even the future characters will be victims of a time lapse and be in new unexpected places. Perhaps it will be in a way that's okay and happy instead of a way that fits with the DOFP storyline and actually acts as a catalyst for the film.
So... how much of what I quoted is impossible to know, because I haven't seen the series?
They can call it whatever they want. But, it is easiest to view this like the normal movie goer will: FC was a prequel (not a new franchise) and this is a sequel to X3 since the original cast is back, but characters from the prequel will appear. That is how the average joe will view it, and by the rule of KISS, it is the easiest way to view it. And, not an inaccurate way, either. Future FC films will just be seen as prequels, anyway. Not a new series.
Generally, people see sequels to prequels as both. If Han and Chewie go back in time to the clone wars... no one's like... "yeah, it's Episode VII!" Especially if their goal in time travel is to prevent the bad version of Episode VII they're living in from happening, especially if most of it occurs in the past. The simplest way to see that film is as a sequel to the prequel, making it a prequel series. Incidnetally, DOFP has always been a sequel to FC, even before people started counting returning cast and drawing conclusions about screen time from that. But we shouldn't preclude the audience's ability to KISS and still see something that continues the story as not a sequel, like Avengers. Even if you disagree that DOFP is like Avengers, don't pretend the audience concludes every story continuation is a sequel.
Anyway, to finish up.
http://www.hitfix.com/the-fien-print/bryan-singer-and-nicholas-hoult-talk-x-men-days-of-future-past-expectations#3qsKDYsIz7CSy3V5.99 said:
"This movie's gonna be not only quite epic, 'Days of Future Past,' but it also takes place in completely different times than the 'X Men' movies have taken place," Singer told me. "There'll be new technology, new things we haven't seen before in 'X-Men' films. Certain characters and certain story and certain drama that hasn't be done yet, so it's not so much sequel. It's more of its own kinda thing."
Perhaps its not possible to know whether the DOFP movie will actually be based on the basic premise of the DOFP storyline (ie, something bad in the future happens, someone goes back in time to prevent it from happening, and succeeds)... but we know it's not a sequel. We know it's set mostly in the time where the X:FC sequel has always been set. We know it was developed, at least initially, as the X:FC sequel. We know it's not called X-Men 4.
It's not X-Men 4. It's not the "#4 movie," it's just the sixth/seventh movie in the franchise, like Avengers.
Is it possible that this precludes a #4 movie from happening? That we'll have to wait and see.