Since his first appearance in 2012 Thanos loomed over the entire MCU, until his eventual demise in Avengers: Endgame.
The 'Mad Titan' kept a watchful eye over all of the goings-on in the MCU from his first cameo, in his hunt for the Infinity Stones. Although each MCU movie had its own villain, everything linked back to Thanos one way or another.
Since his death, however, the MCU has been left without a major threat to its entire universe, with Mysterio attempting to create an "Avengers-level threat" but ultimately failing.
The launch of WandaVision, however, may have opened the door for the MCU's next major antagonist to fill the void left by Thanos.
While many MCU fans have speculated that the likes of Doctor Doom, Galactus and Kang the Conqueror might be the next villains to appear on the silver screen, there is one character that many have overlooked: Mephisto.
Mephisto has had dealings with the vast majority of all of the well-known Marvel heroes in the comics, with his encounters with Scarlet Witch, Doctor Strange and Spider-Man being a few of his most infamous. Introducing him into the MCU would open up many possibilities, including a way of bringing mutants (and therefore, X-Men) into the wider MCU.
In Marvel Comics, Mephisto is the ruler of a separate pocket universe that he names Hell, in order to torment souls that have entered. He is as old as the Infinity Stones, and at one point had his soul split into five fragments and scattered across the universe. Often being considered the devil in the Marvel universe, Mephisto could be connected to a few of the major releases in Phase Four.