Spider-Man: Far From Home ended with one of the best pairs of post-credit scenes in Marvel Cinematic Universe history. The first revealed the return of J. Jorah Jameson, who will be played by none other than J.K. Simmons. The second was the reveal that Nick Fury wasn't Nick Fury at all. The real Fury was on vacation in space the whole time, this was Talos, the Skrull hero from Captain Marvel, doing Fury a solid. But the production nearly made another character a shape-shifted imposter too, as Mysterio was almost a Skrull in Spider-Man: Far From Home as well.
The Fury reveal was both a clever in-joke as well as a bit of a cop-out to cover what was something of a big plot problem. The entire premise of Spider-Man: Far From Home rests on the idea that Fury is taken in by Mysterio. For Peter Parker to be the one who discovers the entire "Mysterio" ruse, Nick Fury has to buy, hook, line, and sinker, that Quentin Beck is from an alternate Earth. Also, there's a giant hole ripped in the universe someplace in the immediate solar system. The head S.H.I.E.L.D. has to believe his people haven't been able to detect it, nor are they able to identify these "Elementals" that keep cropping up out of the blue.
Does that sound like Nick Fury to you? Exactly. Nick Fury wouldn't believe a rose grew from a seed unless he had a timelapse proving it. So, to explain this away, the movie made it not actually Fury.
But what about Mysterio? Turns out the production thought about using the same trick for the film's main antagonist too. Speaking to Collider, screenwriters Chris McKenna and Erik Sommers considered it in the early versions of the script.
There were some early, early versions of this movie where Mysterio was a Skrull… There were a lot of Skrull versions of the story early on. When you’re doing a con artist movie, what we finally landed on — we sat down and talked about how do we keep on fooling the audience, how do we keep on having a lot of fun reveals? How many distractions can we get away with before people want to murder us? [The Mysterio Skrull reveal] was an early idea about why he was doing everything he was doing.
The good news is, the writers ultimately decided it was too easy a cop-out. It also would have gone against what fans know of Skrulls in the post-Captain Marvel universe. In the MCU, the Skrulls are now friends of Earth, which is why Talos filling in for Fury works so well as a joke. Having them suddenly switch back to being bad guys would have confused the audience.
Instead, the writers had Mysterio and his gang be very believable human villains, embittered by their time at Stark Industries and looking for revenge. It's a far more interesting twist and one that sits better with the comic origins. And it leaves the Skrulls to stay S.H.I.E.L.D. allies.