The time traveling/alternate reality jumping Fantastic Four can't reach the 80s?
DOFP's mind traveling has made it easier to make a connection between the universes without it making so they actually enter each other's universe full-on.
Except the only stories that make sense for them to cross over in are The Coming of Galactus (where the world being eaten in the past would stop the FF from ever crossing over), Ultimate X4 (which was underwhelming), X-Men vs Fantastic Four (which requires Shadowcat being injured in a a previous X-Men film and needs to fast-forward the timeline to the late-90s at the earliest), Secret Wars (which is entirely dependent on who owns the Beyonder which means Fox may not be able to do this one), Ultimatum (one of the most hated Marvel stories of all time and requires the FF to break up and most of the X-Men to die), Onslaught (which has the same problem as Galactus) and Betrayal in the Bermuda Triangle (which requires Doom's redemption beforehand and also needs Marvel's permission to use the Scorpius aliens).
So it makes more sense to have the X-Men play catch up with X-Factor and Fantastic Four acting as previews for where the series is headed. I'd save Gambit for after Apocalypse just to not age him too much and set Gambit in the early 1990s. The timeline would work like this.
2015 - Fantastic Four (2015)
2016 - Deadpool (1978), Apocalypse (1983)
2017 - Wolverine: The Lazarus Project (1987), Fantastic Four 2 (2017), Gambit (1990)
2018 - X-Force (1992)
2019 - Deadpool 2 (1993), Fantastic Four 3 (2019), X-Men: X-Cutioner's Song (1994)
2020 - Wolverine: Weapon X, Lies and Videotape (1996), X-Force: Assault on Weapon Plus (1997)
2021 - X-Men: Messiah Complex (1999-2000 New Year), Deadpool 3 (2002), Fantastic Four 4 (2021)
2022 - X-Force: Phalanx Convenant (2004), Wolverine: Final Execution (2005)
2023 - X-Men: Fatal Attractions (2006), Fantastic Four 4 (2023), Deadpool 4 (2009)
2024 - X-Force: Necrosha (2012), Wolverine: Enemy of the State (2015)
2025 - X-Men: Proteus (2017), Fantastic Four 5 (2025), Deadpool 5 (2019)
2026 - X-Force - The Dark Angel Saga (2022), Wolverine: Not Dead Yet (2025)
2027 - Onslaught (This is dependent on whether or not Marvel and Fox can agree to co-produce films. If they can, then this is the team up between the X-Men, Fantastic Four, and X-Force that officially destroys the entire universe at the same time that the MCU goes through the Time Runs Out story to close out Phase 5. A new universe is created from the two merging that keeps the continuity of both but with mild alterations for characters like Viper and Quicksilver.)
Either that or just jump ahead to the OT cast after Apocalypse. Problem solved. And just skipping ahead makes Galactus an event film and Wolverine 3 to be Kitty Pryde and Wolverine.
And if the Spider-Man deal happens, I can see Fox and Marvel working together. Apocalypse will have ended that trilogy and it'll be a clean slate for the X-Men in the present day. With Apocalypse being described as a "final chapter," I can see the present day being a reboot. The only catch is that Cyclops, Phoenix and Storm (probably also Wolverine) will be recast and audiences will just be asked to forget their ages since it's just a film. Just recast and keep continuity to keep them forever young.