As previously theorized (here), the reason why Bryan Singer or Fox aren’t saying whether Halle Berry will be returning as Storm in “X-Men: Days of Future Past” is precisely because they’re still trying to hash out a deal with her to return.
Now Berry herself is saying just that, telling Shadow and Act that she is “practically 90% sure” she will be appearing in Singer’s “X-Men: Days of Future Past” as Storm, the character she’s played in all three “X-Men” films so far, including the first two directed by Singer (and the last directed by Brett Ratner). And unlike some other X-Men characters, Storm was very much alive at the end of “The Last Stand”, so her return is kinda expected.
Also according to Berry, shooting for “DOFP” is expected to start next month in April.
If talks were to break down and Storm doesn’t return, it would come across as odd, but at the end of the day, not a deal breaker in terms of continuity between the two franchises. Remember, parallel timelines and whatnot. When you start playing with that, you can pretty much explain away a lot of things, including missing characters (and characters back from the dead, natch).
Based on the ’80s comic book storyline from the pages of the “X-Men” comic book, “Days of Future Past” would find X-Men from the future journeying into the past to save the present.