I would say that the most likely candidates of villains in future Spidey movies would be:
Lizard - 99.9% sure
Carnage - almost as positive as Lizard, but only because they have already set up the symbiote storyline
Electro - 60% sure (he's something new, and is popular among comic-book fans)
Scorpion - 60% sure(same as above and it'll probably give JJ more screentime)
Venom - 60% sure (he's what fans want and if they decide to bring him back since it's rumored that someone who died in 3 may not be dead)
Vulture - 50% sure (only if Raimi is directing, because he had mentioned that he enjoyed reading about Vulture and he was orginally gonna be in 3)
Rhino or Shocker - 10% sure (if they ever have an opening scene with Spidey kicking some dumbass around)
So, the main villains would almost positively be either Lizard, Carnage, Electro, Scorpion. or maybe even a return of Eddie/Venom. And the only reason I didn't mention Hobgoblin or Demogoblin is because I feel that after Harry, they're gonna be done with the whole "goblin" thing. Hobgoblin's way too mysterious anyway. Was it Roderick Kingsley? Ned Leeds?
Mysterio would be cool but I don't think he should have his own movie. He's not really much of a fighter.
And I remember people saying that villians that appeared in the games based of the movies, but didn't appear in the movie, would never appear in a movie. With Vulture orginally planned for the third movie and The Lizard most likely to appear in the fourth movie, I'd say that was BS.
These villains
The Gibbon
The Grizzly
The Squid
Spider-clone
Jackal
The Walrus
White Rabbit
Stegron
The Spot
Rocket Racer
Hypno-Hustler
Slyde
Big Wheel
Silvermane
Hammerhead
Tombstone
Molten-Man
Razorback
Prowler
Will O' The Wisp
Chance
The Puma
Vermin
Shriek
Kaine
Judas Traveller
Scrier
...The most obscure Spidey foes who will not and should not be in #4.