Avengers: Endgame shook up the Marvel Cinematic Universe, leaving a gaping, Iron-Man-shaped hole in the heroes of this world - and Mysterio thought he could be the one to fill it. There aren't many people in the MCU who could replace Tony Stark, but Spider-Man might be the easiest bet. He's still a newcomer to this universe, but is a flagship hero in the Marvel universe at large, and although he is young, he was also hand-picked by Tony Stark. While there are plenty of other heroes who could become the world's biggest threats, Peter Parker seems to be the plan, and Spider-Man: Far From Home was expected to be the film that set this up.
However, Marvel put a little bit of a twist on this in the film. Rather than seeing Spidey simply step up to defeat the villain and become the new favorite hero, Spider-Man: Far From Home saw him struggling... and wondering if another new 'hero' would be a better choice. And of course, when that 'hero' turned out to be the bad guy all along, Spidey still wasn't able to step into Iron Man's shoes because he's been outed - not just as Spider-Man, but falsely as a villain himself! Now it seems that the MCU is still missing a central hero... and Mysterio may be the one that comes closest to that, for now.
Far From Home Is About Finding The New Iron Man
From the start, Far From Home was about Spider-Man's struggle to become a bigger hero now that Iron Man is gone. Not only is he dealing with grief for the loss of another father-figure, but he's still just a high school kid. The fate of the world feels like a whole lot to carry, especially without Tony (or Steve Rogers, or Black Widow) there to help share the load. Peter wants to continue to help his home, but he's still feeling like a neighborhood hero and wants to find time to deal with all the usual teenage stuff too. Tony Stark may have left him his tech (and his helper, Happy), but Peter just isn't sure that he's the right one for the job... which makes it far too easy for Mysterio to take advantage of him.
When Quentin Beck first appears, Peter thinks immediately that he has found a brand-new mentor. Mysterio is powerful, older, otherworldly, and all too ready to dispense the careful advice. It's easy to see how Peter would want this man to take Tony's place, and he's not the only one. Nick Fury appears to be taken in by Beck as well - and for much of the film, it seems as though the new Iron Man might actually be someone totally out of left field. Until, of course, the big twist is revealed. Mysterio is actually the bad guy, and Peter loses out on a father figure yet again. It would be easy for the film to wrap this up with Spider-Man rising to the challenge, but Far From Home had multiple twists in store, and both the mid-credit scene and some of the tech that Stark left still point to Mysterio as the new Iron Man.
E.D.I.T.H. Makes Mysterio The New Iron Man
Obviously, Mysterio is far from a hero, and he doesn't get through the film alive - but in many ways, this just makes him even more like Iron Man. Peter's glasses are connected to a brand-new AI, E.D.I.T.H., which stands for 'Even Dead, I'm The Hero'. At first, this seems like just a straightforward funny moment; it's typical of Stark to be bigging himself up, even from beyond the grave. This is perfect Stark humor, and a bittersweet moment as Peter laughs at it... and then realizes it will be the last self-aggrandizing joke that Tony cracks.
However, by the end of the film, Mysterio is also a character who fits the 'E.D.I.T.H.' mold. The world saw him as a hero, until he was killed in a big battle - and while the important people know the truth about Mysterio's role in that (and his death), it's likely that many of the public watching would still see him as a hero who died in action. The mid-credits scene, and the use of J Jonah Jameson drive this home; even without the doctored clip that show Spider-Man killing Mysterio, Jameson is the representation of the many people who aren't happy to take all these heroics at face value. In the end, Mysterio seems to have died a hero, while Spider-Man becomes the villain that killed him.
Can Mysterio Return As A Hero In The Future?
With the world (or at least, a chunk of it) believing this new narrative, is it possible that Mysterio may make a triumphant return as the hero in the MCU's Spider-Man 3? It's unlikely, as his death seemed very final, but this is a comic book universe - and Marvel is known for pulling a switcheroo with 'dead' characters. There are plenty of ways that Mysterio could return to the MCU, whether a straight-up resurrection or not. In Far From Home, Mysterio was shown to have a whole team of other villainous types behind him, and they've got some pretty epic hologram tech... it wouldn't be too difficult to see them orchestrate a 'return' as the triumphant hero who cheated death at the hands of the evil Spider-Man. This would certainly make for an interesting way to expand on Peter's inevitable fight to clear his name (and find a way to make his identity secret again). Mysterio may even return as a legacy character; in the comics, there have been multiple 'Mysterios' over the years. A new Mysterio could even appear as an actual hero - a copycat who doesn't realize all the terrible things that Quentin Beck was doing behind the scenes.
However, while a Mysterio return is possible, it may not be the most likely option for the MCU. There are plenty of other ways to see Peter Parker deal with the issue of his revealed identity and the video that makes him out to be a villain, which would allow him to truly step up and become the new Iron Man himself. Bringing Mysterio in as an actual hero (as a reformed character or a new one) would be a confusing arc, especially as fans are looking forward to seeing Spidey rise up and grow into a more confident hero himself, not remain in the shadow of someone else... and bringing him in as a villain would be too much repetition. Spider-Man has already dealt with Mysterio and his duplicity, and it's time to face him off against someone else - like Kraven the Hunter and the Sinister Six. And while Mysterio may not be on his way back, his story and Far From Home definitely did something interesting with Peter's journey to become a full-grown hero, without Tony Stark to help him.