I'm sorry dude, but I disagree with this. Although I don't support doing the film, it can work, and it can work w/o having characters developed in previous films or Spidey being feature. I know it's cliché; but, it is truly about the writing, approach, and direction moving forward.
Yes: the writing, approach, and direction are key things that can make a good movie (and can make ANY movie work in that regard), but randomly shoehorning more characters in films for the sake of saying, "Here's your S6 team!" is pretty damn weak to me.
We had Lizard, but Connors regrets everything he did to Peter, so he wouldn't be with them.
We had Electro, which could work considering all it takes is simple manipulation/motivation for him, so there's a potential choice.
We had Green Goblin, which is a given since he'd be their leader.
Aside from that, the only thing we were given to "other" characters that could potentially be in S6 were Ock's tentacles and Vulture's wings, that's it.
That's one potential character and one obvious character already established, and four remaining that would need to be developed/established in some capacity.
I guess you could use S6 as an origin for the villains, but what would they do? What would be their motivation (aside from Spider-Man)?
The whole concept to a S6 film (without Spider-Man and without pre-established/developed villains to use) just seems silly, IMO.