Jonathan Majors made his grand entrance into the Marvel Cinematic Universe franchise as He Who Remains in the finale of Marvel's Loki series. Majors' role as He Who Remains was somewhat surprising (at first), as Marvel fans thought he was cast as Kang the Conqueror in the upcoming threequel Ant-Man and the Wasp: Quantumania. However, Loki gave MCU fans their first hint of what hardcore Marvel Comics fans already know: Jonathan Majors could appear in any number of roles in different MCU projects, playing different variant versions of Kang. And after seeing Loki there's a growing theory that Majors could indeed be playing a much different character than expected in Ant-Man 3.
Meet Mister Gryphon
In Loki Episode 5, "Journey Into Mystery" Tom Hiddleston's Loki found himself in the Void At The End of Time, a realm where things pruned from the main MCU reality got dumped before being consumed by the Alioth creature. The Void was a treasure trove of fun Easter eggs, as many of the backgrounds had ruined structures or objects that were direct visual references to Marvel Comics and MCU icons.
One of the structures that stood out amongst the ruins was no doubt a ruined version of Avengers Tower, which bore the logo of "Qeng Enterprises." In the comics, Qeng Enterprises was a corporation that bought Avengers Tower from Tony Stark - only to later have it revealed that the company's CEO, Mr. Gryphon, was actually a variant of Kang The Conqueror.
The Gryphon version of Kang wound up being stranded in the 21st century, where he used his future knowledge to built a corporate empire while searching for means to escape the time travel glitch that was keeping him in the modern era. He hired a group of villains to help him further his agenda, and eventually came up against a newer team of Avengers (including Jane Foster's Thor).
The Man Behind Birch
In Ant-Man and the Wasp, black-market middle-man Sonny Birch (Walton Goggins) is the antagonist chasing down Hang Pym (Michael Douglas) and Hope van Dyne (Evangeline Lily), in effort to get Hank's mobile lab and the Quantum tunnel technology inside of it. Birch is supposed to be getting materials for Pym's work, but instead double-crosses Hope and finds a buyer for Pym's lab - for a price of one billion dollars (or more).
Savvy MCU fans have theorized ever since Ant-Man 2 that Birch's buyer would turn out to be a major figure or larger antagonist for the MCU franchise, down the line. A lot of theories pointed to someone like Spider-Man foe Norman Osborn, but after Loki it seems like Mister Gryphon would be a much better guess for who had the money and the burning incentive to get his hands on Hank Pym's Quantum tunnel.
This Is "Quantumania"
With Loki breaking open an entire Marvel Cinematic Multiverse, the Quantum Realm has suddenly become more important than ever as a highway the stretches bewteen and/or across timelines and realities. Quantum energy is a power source that's been teased as next-level in the MCU (see: Michelle Pfeiffer's Jane van Dyne), but it could also be the means by which Kang and his variants move in and out of the main MCU timeline. Loki has primed Marvel audiences to see Jonathan Majors walk onscreen dressed as a simple businessman in Ant-Man 3's marketing and immediately know what kind of larger story is playing out.
Meanwhile, other theories predict that Mister Gryphon will be the bridge between Kang's multiverse conquest in the cosmos, and what has been occurring on post-Endgame Earth in films like Quantumania and series like The Falcon and the Winter Soldier. Julia Louis-Dreyfus' Valentina Allegra is working the shadows and gathering her own 'Dark Avengers' team together - but who do they work for? If their boss is revealed to be Gryphon, it makes Kang a big bad villain on a much crazier level than Thanos.