Iron Man

Peter Parker is one of the smartest characters in Marvel Comics, but that intelligence has not yet been fully explored on film. One great way to do that is to have Parker become the third "science bro," along with Tony Stark and Bruce Banner. Spidey and Iron Man have teamed up multiple times in the comics, and having Robert Downey Jr. show up in the next Spider-Man film would immediately establish the new universe that Spider-Man exists in. It would also immediately catch the attention of moviegoers who may be tired of Spider-Man, but more apt to check the film out if Iron Man performs a flyby or two.