How is Venom after the second movie going ot work? That makes no sense.
Well, they'd have to have a Spidey-gets-the-symbiote after credits sequence. We know a venom room is in ASM2, so it's possible. Not likely, sure, but definitely possible.
The two villains proposed as possible bigger villains than a Sinister Six is the Green Goblin and the Kingpin.
The problem with the Goblin is that he is so ingrained in the Spiderman films, using him also as the big bad for an S6 movie would diminish him. And using Harry to finance the S6 doesn't make a lot of sense since he too is so much as part of the Amazing Spider-Man stories and will likely not make it past TASM3.
Kingpin is an interesting character that was terribly done in Daredevil. The problem is that even with all his resources and power he wound't be able to stand up again the super powers of the Sinister Six. All he can do, since he doesn't have super powers, is send Tombstone to deal with someone. Sorry, but Tombstone would get his arse handed to him by any of the S6 (even Black Cat). Also, if they ever want to do any sort of TV Spider-Man universe series the Kingpin is a perfect villain, so using him in the movies would waste him.
Again, just can't see a Sinister Six movie being good because I can't see any threat from the Spider-Man universe that is more of a threat than the Six!
Actually, the only villain that I could see that could go up against the Six, and possibly be able to take them down, would be...
VENOM!
Now I wonder if that's what they meant. An S6 versus Venom movie. But still, using the S6 in their own movie kills them being used against Spidey, and I want to see that!
Venom is probably the only villain who could fight the Sinister Six by himself in a street fight, but when I say Norman Osborn, I don't mean Norman Osborn on a glider, I mean Norman Osborn with a private army of security with super high tech weapons from Oscorp, a squadron of Spider-clones and enough money and resources to find anyone and their loved ones anywhere. This is a person who could destroy Venom and the Sinister Six from his office desk while drinking scotch with the Mayor. That's the kind of villain the Sinister Six need.
Also, the Sinister Six beating Oscorp doesn't actually diminish the Green Goblin. It just makes him desperate, and more dangerous. Like Loki in the Avengers, him being defeated there didn't diminish him one tiny bit in Thor's franchise.
Kingpin has a lot more than Tombstone on his side. He's got Alistar Smythe, which means he also has an army of Spider-clones if he wants it, and he definitely has a fleet of Spider Slayer robots, and on top of that he hires the best assassins in the world like Silver Sable and Chameleon to take out his enemies, and can also find anyone anywhere because he pretty much owns crime. Just like with Ocean's 11, or Now You See Me, or Inception, these types of villains aren't about a big fistfight in the final battle. They're playing a game of chess with the heroes.
That's why these are the best threats for a team of supervillains, because they have entire organizations that can do a lot more damage to the Sinister Six than one guy who just hits hard, even if that guy is Venom. Money is the greatest superpower, and Kingpin and Norman Osborn have plenty of it. Not only is money the greatest super power, it makes a great target. There are tons of things the Sinister Six might want from Norman or Kingpin: Money, Power, Revenge, Technology, Prestige. But what would the Sinister Six want from Venom? He doesn't have anything they need. Money also makes a great setting. The Sinister Six could go after Oscorp's labs, warehouses, Norman Osborn's penthouse suite, or private islands or Oil Tankers. There's lots of things to do WITH Norman Osborn or Kingpin. The only thing they can do with Venom is get into a fistfight.
The Ultimate comics had Kingpin go away and return better than ever multiple times. If they ever do make a Spidey TV show, they can bring Kingpin back. However, if they don't, it would be a waste to never use him while waiting for a show that may never come (and may not even be possible since Sony gave up the TV rights).
Also, we have no idea how long Harry Osborn is going to last. What do you think would kill him off in ASM3?
And how does using the Sinister Six in their own movie kill them from being used in a Spidey movie? If anything, they need their own movie in order to be used in a Spidey movie.