The Avengers won’t assemble anytime soon. Disney is contemplating whether to send Black Widow to Disney+ following the release of Mulan, and Eternals has been bumped to a February 2021 debut. With the future of the MCU up in the air, there’s no telling when the superhero team will reunite to face another threat.
When they eventually reunite for Avengers 5, they'll need to square off against a villain who puts Thanos to shame. One candidate from the comics could also provide a nifty opportunity to finally bring everyone's favorite mutants into the MCU.
First introduced in 1996’s X-Men Vol. 2 #53, Onslaught emerged as a manifestation of Xavier and Magneto’s intertwined subconscious minds. During a battle between the X-Men and Magneto’s Acolytes, Xavier entered his frenemy’s mind to dampen his powers and stop any further harm. The darkest recesses of Magneto’s mind were transferred to Xavier’s, merging with the telepath’s very own darker attributes. And so, Onslaught evolved into a foe powerful enough to take on a physical form outside of Xavier’s mind.
As the unexpected brainchild of two of the world’s most powerful mutants, Onslaught had immeasurable and catastrophic abilities. He could mentally control anyone he wanted (including cybertech machines like the Sentinels), warp reality, manipulate magnetic fields, and make a fight with the Hulk look like child’s play. He also looks like Magneto's final evolution in some terrifying Pokémon game.
At one point, Onslaught attempted to establish Xavier’s vision of peaceful human-mutant relations, but Magento’s destructive tendencies moved him to wage war against humans. It was utter chaos for a while. Onslaught even created a second sun to destroy his enemies, and only the combined efforts of the Avengers, X-Men, and the Fantastic Four could defeat him.
There's still no clear indication how the X-Men will be introduced to the Marvel Cinematic Universe. However, Onslaught’s arrival would be the perfect way to bring them into the fold and effectively launch a mega-crossover movie. After all, Onslaught is an X-Men villain first and foremost, but the fact that he nearly killed both factions sets him up to be even more of a menacing threat than Thanos.
Onslaught could even be introduced as early as WandaVision. Perhaps whatever mind-warp Scarlet Witch undergoes will further Onslaught's manifestation in the real world. Scarlet Witch was a mutant in the X-Men comics, as well as Magneto’s daughter, so it’s possible that she will work as a stand-in to introduce Onslaught before he grows strong enough to create a corporeal form in time for Avengers 5.
Technically speaking, fans are already familiar with the X-Men (following a string of movies that ended with New Mutants), so it wouldn’t take much for the MCU to softly reboot the characters and bring them in to fight an omnipotent villain like Onslaught. Weaving in Scarlet Witch to help launch the antagonist’s storyline and the X-Men would be a massive undertaking that Marvel may not be ready for yet, but that's never stopped them before.