Your story has potential... I see your point with Fury and the Avengers... just need to expand on it a bit more... I don't like the idea of doing Hopgoblin in one movie... you have to flesh it out over two if not three movies... that means Kingsley in SM4/5 to begin with... major buildup would be required as you pointed out, and thats not even including Octavius. And you can't get in the way of your main stories... Lizard/Scorpion for 4/5... the writers won't have an easy time with this I'll give you that... but I'll take my chances with a Sinsiter 6 rather than Carnage, or Morlun, or Hopgoblin as a stand out. Plus you still need to throw in Electro unless you are keeping Scorpion alive... but we can't make SM 4/5 without killing any bad guys. I'll try to think of a King Pin S6 version later on... my unfinished script for SM4 hints at all of this... not sure if I am gonna finish it though
First try:
How about if we see him approach the villains toward the end and recruit them. That would give us a cliffhanger to end each movie. This would keep him in the shadows (much like the early issues of the Hobgoblin), and make the audience wonder what is happening. Like, approach Kraven at the end of Spider-Man 4, and Mysterio at the end of Spider-Man 5 (still undecided on Scorpion on the team). Gives Kingsley an aura of mystery. But he doesn't free them yet. Then, in Spider-Man 6, we can see him approach Sandman and use his daughter to recruit him. Then, he can reveal that Ock is alive (call him the final peice), and gets Ock to join, perhaps by getting the tenticles to take his mind again). If I went the Electro route, we can have Electro be the villain of the first act. He gets recruited shortly after his defeat (he is nothing more than a thug...but feels small in the team). Hobgoblin becomes known when he busts them out of jail. Spider-Man is later drawn out by Hobgoblin (after some sort of team meeting of course). They talk a little, and then the rest of them come in and the Sinister Six battle Spider-Man, and dispatch of him easily (each giving a "remember me" type line, of course). As Peter is captured, he tries to reason with the Sandman and Ock. Asks for their help. Ock doesn't, but Sandman looks conflicted. Hobgoblin commands Ock to finish him, and he gets angry and says something like "I answer to no one". They then argue and fight. Then one of the others goes for the kill on Spidey, and the Sandman stops them. Spidey and Sandman team up, dispatch of them, but Sandman gets defeated in the process (maybe by Electro since it would be harder for Mysterio or Kraven to beat him), who is then taken out by Spidey. Hobgoblin defeats Ock, is about to kill him, when Spider-Man stops him. Now, it is down to them. They have a brutal battle, and Spider-Man wins, and the Hobgoblin seemingly dies (but you don't see him die).
I say one very long sequence is sufficient for the Sinister Six, and having Electro established early in the film develops him. Also, recruiting of Kraven and Mysterio in the other movies gives us interest in Kingsley and saves time spent on recruitment in SM6. Recruiting scenes for Sandman and Ock are sufficient, as they are developed already. One long fight would be about the extent we could do with them because, lets face it, the Sinister Six would be very expensive and doing one long sequence would save a little money.