X-Men: Dark Phoenix is going to be a game-changing chapter of the X-Men movie franchise, arriving at a time when the franchise itself may be headed for drastic changes, thanks to a deal in the works between Fox and Disney. However, with there still being so much about Dark Phoenix we don't know, there could be some real twists and surprises that are game-changers in and of themselves.
One theory (posted over at Screen Rant) now predicts that there could even be one or two more iconic X-Men villains that could be making a debut in X-Men: Dark Phoenix - with the biggest shocker being a potential appearance by Onslaught!
So who is Onslaught? Well, he's a "psionic entity" (i.e., a being made purely of psychic energy) who was created by a violent merging of minds, after Professor Xavier forcibly wiped Magneto's mind, leaving him in a vegetative state. That act left some of Magneto's psyche imprinted on Xavier's subconscious, and that merging grew into a third personality, called Onslaught. Onslaught secretly terrorized the X-Men universe for an entire year before anyone figured out his identity, leading to a battle with the X-Men, Avengers, and entire Marvel Universe.
Onslaught started with the powers of both Professor X and Magneto, eventually adding the reality-altering powers of young Franklin Richards (son of Fantastic Four's Reed Richards and Sue Storm), and the all-powerful psychic abilities of Nate Grey, an alt-universe version of Cable. He's as powerful a villain as they come, and the nature of his origin and storyline would be a timely fit for X-Men: Dark Phoenix.
How Onslaught Could Fit Into X-Men: Dark Phoenix
We know that X-Men: Dark Phoenix will re-shape the classic Marvel Comics storyline into a tale of how Jean Grey becomes of the Phoenix during one of the X-Men's increasingly dangerous missions as world heroes. It's stated that the reason for the X-Men's heroic campaign is the "growing ego" of Charles Xavier, which pushes the team into the tragedy that unleashes Jean's Phoenix powers.
The particular wording is something fans have seized on, predicting that Charles' "growing ego" could be a foreshadow of how Dark Phoenix sets the stage for his transformation into Onslaught. In the comics, it's Charles' hubris in playing god with Magneto's life that created the devil Onslaught; in the movies, however, we could take a much different path to the same destination.
There's been a very grim death teased for Dark Phoenix, and a first-look photo hints at that death occurring, with rumor suggesting that it will be one of the four remaining original characters from the First Class continuity (Xavier, Magneto, Beast, Mystique). Since Xavier is in the photo, we can rule him out - and since Beast is seemingly in it too, there's reason to believe that Mystique and Magneto are the ones with their heads on the chopping block.
Our best guess is that it could be Michael Fassbender's Magneto who dies, after trying (and failing) to corral Jean's Phoenix power. That death would open the door for a big twist on the Onslaught origin: instead of using his power to wipe out his friend's mind, Charles could use his gift to house some of Erik's psyche, before his friend's physical body died. That would lead to a third act where the Charles/Erik hybrid manifests power that could rival Dark Phoenix, in the sort of climatic battle you would expect in an X-Men movie. It wouldn't be the version of Onslaught from the comics, but it would be the sort of (superficial) adaptation that Fox's X-Men movies have been known for.
If that sounds like another opportunity for the franchise to squander, then maybe you'll feel better about how Onslaught could be the key to something that Marvel fans have been waiting for, all along...
The X-Men Movie Universe continues with The New Mutants on April 13th; Untitled Deadpool Sequel on June 1st; and X-Men: Dark Phoenix on November 2nd.