Imagine an X-Men movie universe scenario in which MCU Thanos had snapped his fingers. A bunch of events have unfolded, similarly to the MCU, that sees Thanos killed, then reemerging from the past in 2023.
A battle is taking place outside a destroyed X-Mansion between Thanos and the trio of Wolverine, Storm and Iceman (replacing Captain America, Thor and Iron Man).
Thanos wins and Wolverine is the last man standing, as the Mad Titan's army arrives. The clawed hero with the regenerative healing factor approaches the army, still determined, still naively hopeful.
But then he hears a voice on his communicator:
"Logan, do you read me?"
He stops in his tracks.
"Logan, it's Jean".
Jean had died five years earlier.
Suddenly, portals open behind him...
Out of one comes Jean, Professor X (let's say his wheelchair can fly) and Cyclops.
Out of another comes Cable, Deadpool, Domino and Firefist.
Out of another comes Colossus and Negasonic Teenage Warhead.
Out of another comes Magneto with an army of 100 Sentinels.
Out of another comes Darwin, Banshee and Angel Salvador.
Out of another comes Bishop, Blink, Sunspot and Warpath.
Storm and Iceman awaken from being K.O.'d.
From under the rubble of the X-Mansion, Quicksilver, Mystique, Nightcrawler, Angel, Kitty Pryde and Beast emerge.
Thanos' army is exactly the same, except he has Fox's Apocalypse on his side (the bad guys need some TP).
Any X-characters who've been played by young and old actors are composite.
So...
X-Team (all live action)
Wolverine
Storm
Iceman
Jean Grey
Professor X
Cyclops
Cable
Deadpool
Domino
Firefist
Colossus
Negasonic Teenage Warhead
Magneto
100 Sentinels (Under Magneto's control)
Darwin
Banshee
Angel Salvador
Bishop
Blink
Sunspot
Warpath
Quicksilver
Mystique
Nightcrawler
Angel
Kitty Pryde
Beast
Vs
Thanos' Army (all live action)
Thanos
Apocalypse
Ebony Maw
Cull Obsidian
Corvus Glaive
Proxima Midnight
The Chitauri (foot soldiers, space whales, gorillas)
Outriders
Sakaarans
Scenarios:
1) Thanos gets the Space and Mind Stone only.
2) Add the Power Stone on top of those two.
3) Add the Time Stone on top of those three.