The rumor started with Daniel RPK, a well-known Insider for Marvel Studios:

If you thought the Infinity Saga was the most ambitious crossover event in the history of cinema, wait until Secret Wars arrives. In Marvel Comics, Secret Wars was the result of an Incursion – mentioned in Doctor Strange in the Multiverse of Madness, as two universes merge into each other, causing a cataclysmic event.Secret Wars also included a tremendous number of characters and variants which seemingly gives an opportunity to allow more actors to play characters who haven’t surfaced yet in the MCU or allow former actors to reprise their beloved roles. If that’s the case, Tobey and Andrew would undoubtedly remain the top choice as fans are surely anticipating the next reunion for the three Peter Parkers following last year’s Spider-Man: No Way Home.For now, Marvel Studios hasn’t yet announced who is returning to the next Avengers films. Certainly, the next Avengers crossover would be more massive than the previous Avengers: Endgame.Spider-Man: No Way Home – The More Fun Stuff Version will premiere in theaters on September 1.

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New MCU Leak Suggests Tobey Maguire and Andrew Garfield s Return in Avengers  Secret Wars - 4