One thing I just thought about is the difficulty of this within the whole timeline.
For example, the current X-Men movie is going to be set in 1983. The expectation is that there will be a newly casted Jean, Scott and Ororo; and hopefully Nightcrawler as well.
Add in the staples of Mystique, Magneto, Beast and Charles; and it's pretty obvious that they will likely continue forward with these actors and actress and characters. Fassbender, McAvoy, Lawrence and Hoult are popular, so why would they ever think of moving away from them.
The other option would be to go back to the OT cast, but with the timeline of these movies that's not very likely. A good number of the OT cast is aging out, so we're unlikely to ever see them again.
So, what is the expected timeline of this Fantastic Four movie?
If it's in the past so they can hang out with the X-kids, is that really the best setting for the F4?
And if it's in the present then how exactly would a crossover work when the more popular X-characters (and actors) are aging out or are stuck in the 1980's?
It's a bit of a dilema, if you think about it...
It seems that they want what the Avengers gives, or what Justice League may give, but here's the problem... Both of those cinematic universes are set in the present, while the currently more popular X-Men are not. So are they willing to place the F4 in the past just to be able to do a crossover with the X-kids, when the whole point of the Ultimae F4 is to make them more modern in present day New York? Seems unlikely.
And really, the only benefit to doing this is to be able to have Franklin in a present set movie. And since Franklin's powers are too powerful, then a fair question is why. Why do that?
So the logical options are as follows:
1. Set this Fantastic Four movie in the 1980's or 1990's just so they can have a crossover movie with the X-kids.
2. Set this Fantastic Four movie in the present, which makes much more sense, but now they can only crossover with the OT cast, which are (at least some) aging out. Guess they could have cameos of some of the younger OT cast, like Rogue, Bobby, Kitty, etc.; but without the main characters that would be a pretty pitiful "cross-over".
3. Just do their own thing with no corss-over. This would effectively be admiting that the X-Men movie-verse is so convoluted as far as the timeline that trying to fit the F4 into it is just too difficult, and unfortunately that is something I agree with.