Chameleon being a secret villain would fit in nicely with the character so that would be fine by me. I'd like to see the Sinister Six try and pass off Spider-Man as one of them by having Chameleon disguise himself as blue/red-suit Spider-Man and frame him for various crimes. At first the public would buy into it thinking Spider-Man is a member of the Sinister Six until the real Spider-Man bonds with the symbiote and shows up to confront his imposter in public (see Spectacular Spider-Man). Eventually he exposes Chameleon who wouldn't be identified during the film until he's unmasked by Spider-Man. It could be fun for the audience to try and figure out and would be a good nod to fans of the comics and pay homage to ASM #1.