Fox released a lineup of their new products of films and one read:
Untitled Fox/Marvel film scheduled for wide release on July 13, 2018
Now, I know some of you are probably thinking it's not Onslaught, and it's true. I mean, Deadpool needs some love after X-men Origins: Wolverine.
Let me tell you the 2 reasons on why it is probably Onslaught.
1) Money. (Isn't that the reason for everything.)
For starters, the story itself, if you read the comics, has X-men and Fantastic Four. Both properties are owned by Fox. They could Avengers-ify those properties and get a lot of money. Onslaught is probably their easy card. They're using both Days of Future past and Apocalypse. The used both moneymakers and frankly, it's their last resort.
2) It just makes sense.
Let me tell you what Fox should do. Now, since [X-Men: Days Of Future Past](movie:203942) changes the timeline, they can reboot the entire franchise. A key player of Age of Apocalypse is Legion, who is the son of Xavier in the comics, he goes back in time and kills Xavier, in the comics. He doesn't do it intentionally though, he goes to kill Magneto but he goes to when they're friends and kills Xavier altering the timeline. Well, we know that in Days of Future Past movie. Wolvie, Quicksilver, and Xavier rescue Magneto from his prison and obviously, if a guy breaks you out of prison you're buds. So, at the end of DOFP X and Magneto are friends. BOOM!!! Legion kills Xavier. End DOFP. Then, Age of Apocalypse comes and Xavier is back. Yay!!! But he's haunted by visions of his death. Leading to Onslaught being born. Also, instead of Jean Grey they could use Emma Frost. Frankly, that's what I think.