Spider-Man has always played a surprisingly large role in Marvel's Secret Wars. The original miniseries from 1984 first introduced his black costume -- and, by extension, Venom -- while both Peter Parker and Miles Morales played key roles in the 2015 reboot. The latter version is expected to serve as the basis for Avengers: Secret Wars when the Marvel Cinematic Universe delivers its version of the vaunted storyline, with the Multiverse and its myriad dangers already an established part of the saga. Spidey certainly has copious experience with the concept on the big screen, particularly with Spider-Man: No Way Home and the animated Spider-Verse features hinging on Multiversal incursions.
No Way Home may hold a vital key to the MCU's version of Secret Wars, as evidenced in a recent Reddit theory posted by user Handsart. That film ends with Peter Parker erased from everyone's minds -- including his friends and allies -- leaving him to continue as Spider-Man alone. According to the theory, it may provide him with a unique perspective that could prove vital once Avengers: Secret Wars arrives.
Secret Wars Involves Multiversal Incursions
2015's Secret Wars recounts how a series of Multiversal incursions threatens to destroy all of reality. It ultimately results in a single world, Battleworld, composed of the remnants of multiple realities and ruled by Doctor Doom. Survivors from Earth-616 (the "core" Marvel comic-book universe) and Earth-1610 survive as well. Some of them remember what happened before Battleworld came into existence, and they fight to overthrow Doom and restore the Multiverse. Their ranks include Parker and Morales, who play large roles in the story.
The MCU has a ways to go before that and will need to integrate its own timeline into whatever version of the story it delivers. Doctor Doom, for instance, will almost certainly be replaced by Kang the Conqueror since Doom has yet to formally arrive in the MCU. Considering Spider-Man's importance in the story, however, it's hard to conceive of Secret Wars without him. Luckily, the events of No Way Home provide a strong narrative hook, as the Reddit theory notes.
The MCU's Spider-Man Is a Perfect Leader for Avengers: Secret Wars
No Way Home entails an early Multiversal incursion of the kind expected to arrive with Secret Wars. That results in its big draw of all three cinematic Spider-Men appearing onscreen together, as well as multiple villains from previous Spidey movies crossing over into the MCU. Doctor Strange ultimately resolves it by making everyone forget that Peter Parker ever existed, restoring the respective timelines, but leaving Peter a stranger to his friends MJ and Ned.
According to the Reddit theory, that could render Peter immune to the effects of Multiversal incursions. Presumably, he'll be resistant to any changes to perception and memory that come with future incursions, including a cataclysm like the creation of Battleworld and any other incidents involving Kang that precede it. That leaves him set up not only to serve as an effective audience surrogate but also someone who can rally the other Avengers to his side and undo whatever calamity has affected the Multiverse.
As the theory notes, Marvel's House of M storyline adopted a similar conceit, with Wolverine the lone figure who understands what's happened. Secret Wars could even represent the moment when he and the X-Men crossover into the MCU. Regardless, however, Parker's status at the end of No Way Home is an opportunity to bring him into the forefront during Secret Wars, possibly as the hero who leads the Avengers to victory. There's far more story to come, but the theory looks to be a durable one as Phase Five continues moving forward.