Thematically name one way Mysterio and his story enhances the story of the personal troubles of Peter Parker? How is he a good foil, how does he teach Peter a lesson, how does make Peter grow as a person on a deeper level? We need an Ock or Goblin.
Mysterio could work if the movie takes a serious look at the burden being a superhero brings upon Peter. Spider-Man is the reluctant hero, while Mysterio is the frustrated omega wishing to be the alpha. It could be interesting to see Beck realize the responsibility that comes with the increasing power he gets and that ultimately leads to his downfall. Honestly though that would take way too much time on his origin.
I'd like to see Mysterio be like Scarecrow in The Dark Knight with a brief defeat from Spidey, while he faces a larger and more established villain in the main plot.