I don't believe either could carry an entire flick. Scorps would be good for having as a second villain along with maybe Jackal. As for Rhino, I wouldn't mind seeing him tangle with the Wall Crawler like kicking a sequel off, but Spidey would have that fight wrapped up in under ten minutes, methinks.
Still yet MORE news from this topic. From Super Hero Hype:
Superhero Hype! has learned exclusively from a very reliable source that Spider-Man screenwriter David Koepp is in talks with Sony Pictures
to write Spider-Man 4!
Our source tells us that the studio is really excited about Spider-Man 3 after having seen a first cut of the movie. Add to that the phenomenal reaction to the screenings of footage at the Consumer Electronics
Show (CES) in Las Vegas a couple of weeks ago and the momentum leading up to this point from last summer's Comic-Con, which have sparked the studio to approach Koepp about penning the fourth film's screenplay.
Koepp is not signed yet, but the studio is definitely moving forward with another installment.
You are correct about his contract. I suspect rather strongly they will dangle the carrot in front of him to get him to sign on for at least one more. At this point in the series, I really can't see him being replaced with another plausible actor.