I'm really intrigued by the idea of the Sinister Six being featured in a movie. They are, in my opinion, THE best supervillain team in comics. I've loved the whole concept going back to when I was first introduced to it in the "Insiduous Six" episode of the Spider-Man cartoon. The "Return of the Sinister Six" story from the early 90s remains one of my favourite Spidey comics, and their return in "Ends of the Earth" was enough of an event for me to pick up Amazing Spider-Man when I hadn't been a regular reader in years. I loved how in particular that most recent appearance of theirs presented the team as formidable enough to be a challenge for The Avengers, rather than just a threat to Spider-Man.
As far as the movie lineup goes, here's who I would include.
1. DOCTOR OCTOPUS
I think he's essential for a Sinister Six story. There have been a few short-term variations on the group that existed without him (like the Sinister Seven formed during the Clone Saga to avenge his death), but there's a reason those versions are always the least memorable incarnations. Doctor Octopus is the beating heart of the group, the genius and master strategist who can unite all these street-level thugs and utilise them in a way that elevates them to global threat. If the third film is going to be all about the Osborns with the Green Goblin as the primary villain, then Doc Ock is the guy I'd save until the 4th film, or have him lurking in the shadows recruiting members to his team throughout the 3rd film. There's the problem of Alfred Molina doing a pretty much definitive take on the character in the last franchise, but I think there's enough angle to do a different portrayal: make him cold-blooded and ruthless were Molina's take was tormented and sympathetic.
2. ELECTRO
This all depends on Jamie Foxx's Electro surviving in The Amazing Spider-Man 2, but if he does, his inclusion in the group is a no-brainer. Electro has been in just about every incarnation of the group (unless I'm wrong, every comic version of the group, with him being replaced with Shocker in the cartoon's Insidious Six), and he adds some major firepower to their ranks.
3. RHINO
Another no-brainer. He'll already be established in Amazing Spider-Man 2, so no need to spent time on an origin. And if you can get Paul Giamatti in your film, you put him in your film.
4. MYSTERIO
Now, unless I'm mistaken, Mysterio has been in EVERY version of the Sinister Six. I also think he's an awesome villain, one of my favourites in Spidey's rogues gallery. But his inclusion here is a conditional one. Namely, it'll only work if, as I'm guessing, the "man in the shadows" played by Michael Massee in the post-credits sequence of the first film is Mysterio. That way, he's already an established, existing character in the universe and doesn't need to be introduced from scratch.
5. VULTURE
Similarly to Mysterio, this only really works if, as rumoured, Colm Feore has been cast as Adrian Toomes. If he's an existing character in the universe, it won't take much of a nudge to then cement his role as the Vulture, be it in the 3rd or 4th film.
6. THE LIZARD
This is the one I'm least certain on. Green Goblin, or Black Cat, or someone new could be feasible candidates for this final slot, but I think Lizard just takes the lead. On one hand, it makes sense, as he's an already-established villain in the mythology. But I don't know how Curt Connors would slip into the team dynamic. Perhaps something is done to compel him to ally with the group against his will? But honestly, I could just as likely see The Lizard allying with Spider-Man and maybe Black Cat to battle the Sinister Six, as opposed to being one of the bad guys.
So, that's 6 villains, and only one of them who won't already be to some degree established by the second film. If you opt to bring in another new character to be on the team instead of Lizard - say, Scorpion, or Chameleon - then that's still only 2 new characters you need to introduce over the course of the 3rd and 4th film to give you a complete roster. Not too daunting a task.
