^ In thinking about the anti-hero concept I must agree with Morbius.
Another example is the movie Mobsters where the underdog, charismatic mob group is just four friends who go up against the two heavy hitter Dons.
See that one works because they must use brains over brawn (just like in any of those other villains versus bigger villains stories) and because the "good guys" are the underdogs.
Problem is, who would the Sinister Six be considered the underdogs against?
If it was the Sinister Six versus a hero (like Spider-Man) or a group of Heros (like the Avengers) then the hero or hero group would be the ones the general audience roots for. If it is a group of Anti-Heros (which is unlikely to begin with) then who would be the high threat villain or villain group?
I guess what I'm saying is that the Sinister Six is almost the epitome of the Spider-Man villains. Having to overcome 6 high end villains is almost an impossibility, making the story exciting. Same with the Maximum Carnage story where Spiderman needed help from Black Cat, Morbius, Iron Fist, Captain America, Firestar, and even Venom in order to beat Carnage, Shriek, Hobgoblin, demogoblin, etc.
So, take an S6 group of Doc Ock, Vulture, Rhino, Electro, and two others (to make people happy, say Mysterio and Kraven); and now tell me which existing villain would be more of a threat. Likely you would need a group of higher level villains, and I just don't see there being any more of a threat than the S6!
Also, the idea of having the S6 shake out over 2 Amazing Spider-Man films doesn't mean there wouldn't be any fighting by Spider-Man in the first one. Of course there would be! All I am saying is that the majority of the characters (maybe not all) would need to be introduced in the first flick and then brought together as the threat in the second. In some ways this may be happening already...
If the Goblin is the one to bring them together, then we already have this in place...
Goblin, who is already introduced, could be one of the 6, even though many do not like that idea...
Rhino, who is already introduced, could be one of the 6 as the muscle...
Vulture, whose wings will be shown, could be introduced in an after credits scene with a character already introduced in TASM2 (Adrien Toomes, or somebody else)...
Doc Ock, whose arm harness will be shown, could be introduced in an after credits scene with a character already introduced in TASM2 (Otto Octavius, or somebody else wearing a fedora or working as Oscorp)...
And then, as I have been arguing for months, they could use some "villains" to fill out the six, that aren't really villains. Black Cat, who many believe the alter-ego of which will be introduced, could easily fill this bill by switching sides and helping Spidey; as could Curt Connors, who has already been introduced.
See, there is a way to do the S6 in one movie, without having the too many villains issue!!!!!