If its anything like X-Men and Fox, they have the rights to anything and everything Spidey related including villains and supporting characters etc, which would more than likely include all Spider related characters.
I think doing a movie without Parker is gonna be tough. Like Batman and Superman, Spidey is one of the few characters who has a secret identity that is known by just about everyone, regardless of whether they ever read comics or not. Putting anyone other than Parker in the mask is gonna cause confusion. Lesser known characters in Guardians works, but Spidey is not lesser known, so changing the secret id is gonna be a big deal.
With Iron Man they could have stuck Rhodes in the costume. With Green Lantern they could have stuck Gardner in the costume. etc etc, as their identities were not as well known to the general movie going public, but could they get away with rebooting Iron Man and replacing Stark? I think that would be a tough ask, even with Rhodey being in the films.
Personally I will be sad to see Garfield go as he was head and shoulders above Maguire for understanding the character, and the first film for me was better than any of the Raimi ones, and I really liked those films (I even tolerated #3).
The second AS suffered from the same problem as #3 in the original trilogy. Too bloody many characters, and it was pretty much set up to help the spin off movies that were coming.
Oh, and Stone made a much better Gwen than Howard in #3, and was far superior to Dunsts MJ.
