Since the CW's Crisis on Infinite Earths crossover event, Marvel and DC have begun to experiment more with the multiverse as a way to merge past and present franchises into one huge superhero blockbuster. Audiences have already gotten glimpses of these efforts with the trailers for The Flash and Spider-Man: No Way Home. While there's a chance that they will be exemplary films, studios take great risks with having past actors reprising iconic roles in someone else's story.
Though the CW's Arrowerse already introduced the multiverse long ago, it finally reintroduced past film characters like Brandon Routh's Superman and Ezra Miller's Flash to the spotlight. At the same time, the Season 1 finale of Disney+'s Loki introduced the existence of the multiverse to the Marvel Cinematic Universe, giving it the opportunity to bring back past Spider-Men and even Fox's X-Men. This will allow the studios to bring back fan-favorite characters in some interesting stories, but they might have a negative impact on the overall quality of their films.
Inserting so many other heroes and villains risks diluting the main hero's story for the sake of brand recognition (this has even been the case with having Iron Man appear in Spider-Man: Homecoming). Having Michael Keaton's Batman and Tobey Maguire's Spider-Man will no doubt attract plenty of audiences, but studios should first and foremost focus on creating a satisfying and focused narrative for their main protagonists that allows them to grow on their own.
Likewise, cinematic universes have become harder to follow, with audiences having to watch multiple other films to catch up on what's happening in new movies that use past plot elements. If Tobey Maguire and Andrew Garfield are indeed starring in No Way Home, moviegoers unfamiliar with their characters would have to go back and watch five films across two different continuities to understand them and their stories.
The Arrowverse and Sony's Into the Spider-Verse found a way around this issue by using the multiverse to introduce entirely new characters into their main franchise. This doesn't rely on audiences having prior knowledge of these newcomers' past projects to understand their part in the story. They also introduce the possibility of them having their own spin-offs that don't intersect with the main continuity, making them easier to follow.
The concept of the multiverse also allows studios to revisit old film franchises and give audiences closure with more satisfying endings (examples include Batman's Burtonverse and Andrew Garfield's Spider-Verse). However, if studios want to rectify their mistakes with their film franchises, they could just continue making films with them with their heroes in their own universes. But the fact that they're adding them into other characters' films implies they don't have enough faith in those films being able to stand by themselves.
The reliance on these past franchises is also an example of Hollywood's tired trend of using nostalgia to market their movies, which prevents studios from taking risks with a new cast and story. Though this nostalgia trend has usually come in the form of sequels, reboots and remakes, these multiversal crossovers act as sequels for the characters they bring back. Unfortunately, creating so much hype for the return of such beloved characters sacrifices quality for quantity, as it can distract filmmakers from creating good stories for the new characters they're supposed to focus on.
All in all, multiverse films open the door for exciting and unique stories to be made with characters old and new. Despite this, navigating such a vast premise can lead to some massive pitfalls for the studios due to over-reliance on nostalgia and saturating their story's main narrative with characters from other franchises.