In July 2022, Kevin Feige announced at the San Diego Comic-Con that Avengers: The Kang Dynasty and Avengers: Secret Wars would serve as the two films to conclude Phase Six of the MCU. The former film's title obviously gave away the name of the main antagonist, but the latter's villain has been up for speculation until recently.
According to internet rumors, The Beyonder will be the main antagonist of Secret Wars. A cosmic entity, The Beyonder is described as the sum total of an entire multiverse called the Beyond-Realm, or simply Beyond, hence the name.
In Marvel Comics, the title Secret Wars has been published in two different comic events. In the first version, the Beyonder observes the mainstream Marvel universe and is fascinated by the presence of superheroes on Earth and their potential. He chooses a group of heroes and supervillains and teleports them against their will to Battleworld, a planet he created in a distant galaxy. The Beyonder then proceeds to pit the two groups against each other for his entertainment.
In the alternate version of the comics, Doctor Doom refuses to fight the heroes and attacks the Beyonder, stealing his abilities and essentially becoming a god.