Too many people want to see in 3rd and 4th movie(Green Goblin, black suit, Venom, Sinister six). Well, this is too much! Black suit and Venom can´t be done in a Sinister Six movie. Also, Green Goblin and Gwen Stacy death also can´t be done in a Sinister Six movie. This needs a solo movie.
Here is my idea on how Sinister Six should be done.
TASM. Dr. Curtis Connors is back normal, but in jail.
TASM 2 - Aleksei steals Oscorp truck, big Rhino suit inside truck. Aleksei turns to Rhino (Mechanical suit), but Spider-Man beats Rhino. Rhino is now in Ravencroft Prison. Now Max turns to Electro, Spider-Man defeats him, Electro is also now in Ravencroft Prison. Adrian Toomes owns Ravencroft prison. Adrian Toomes and Norman Osborn are business partners.
TASM 3 - Norman Osborn uses serum, turns back to normal, but Norman Osborn is stronger and faster. Adrian Toomes is an engineer who developed a magnetic air transport. After being wronged by Norman Osborn, Toomes takes on the moniker of the Vulture to seek revenge on him. (NORMAN OSBORN STOLE GLIDER AND SUIT.) We see Vulture and Green Goblin fight. Gwen Stacy still works in Oscorp, finds horrible truth, tries to tell Peter, but Green Goblin kills Gwen Stacy. Spider-Man kills Green Goblin.

TASM 4 - Gwen Stacy is dead, Peter Can't Live without Gwen Stacy. After shocking news that Norman Osborn was Green Goblin, and Spider-Man killed him, then JJ Jameson gives a big award for people, who finds out who is Spider-Man. Kraven The Hunter agrees with the challenge. Kraven is defeated by Spider-Man, Kraven is now in Ravencroft Prison. While Gwen Stacy is dead, and Peter Can't get over of death of Gwen Stacy, then new villain Mysterio tries to make Spider-Man crazy, showing deaths, Illusion, but Spider-Man is smart enough to beat Mysterio who is sent to prison. Also, in the movie, Spider-Man has the black suit, he found this from Oscorp. Spider-Man removes the black suit, and hide this back in Oscorp.

TASM 5 - Harry Osborn is mad. His father is dead, he promises revenge. Harry finds out more about black spider-man suits, and finds out that Symbiote is behind this. Harry uses Symbiote, turns to Venom. This movie is all about Harry Osborn / Venom, vs Peter Parker / Spider-Man. We see more Mary Jane. Peter Parker new girlfriend is Mary Jane Watson. End of movie, Harry Osborn dies with Symbiote.
TASM 6 - Doctor Octopus is almost ready with his work, but because Norman and Harry Osborn are dead, then Oscorp will be shut down and closed. Doctor Octopus dosent want Cancel his work and he decides to test his unfinished work. Something goes wrong, and he turns to Doc Ock. Doc Ock and Spider-Man fight, but Spider-Man defeat Doc Ock, and is sent to Prison. Now having Rhino, Electro, Kraven the Hunter, Mysterio, Vulture and Doc Ock in prison, they make escape plan. They break Ravencroft Prison and escape.
They know that Spider-Man sent them into prison, and they will be defeat again 1 by 1, but they make one big team to defeat Spider-Man.
Sinister Six is easy to beat. Sinister Six is not balanced team. Vulture wants to own Oscorp, Doc Ock wants to have his work back, Rhino is just a criminal guy, Electro wants to destory world, Mysterio just wants to play, and Kraven the Hunter wants to have his second chance to beat Spider-Man in fair fight. They make their own stupid things, and fails to beat Spider-Man.
Well, end of movie, Spider-Man defeats all villains, Peter Parker and Mary Jane Watson marry, end of movie
Important info:
Why Mysterio in TASM4? Because it´s the best villain after Gwen Stacy's death. He can make Peter/Spider-Man's mind crazy.
Why Harry Osborn is Venom, not Eddie? 1. We saw Eddie in Spider-Man 3, we don´t need to see same story over again. People complaining ripping off Raimi Spider-Man, so, now it´s all different. Also, if Eddie is Venom, and Harry Osborn is New Green Goblin, then it takes too many time to have Character Development, and then we wouldn´t see Sinister Six. Then we would need 7 Spider-Man movies to have and see Sinister Six. Symbiote never came from Space. It was made in Oscorp.
Dr. Connors is not part of Sinister Six. Why? Anyone can use serum and turn to Lizard. We don´t need Dr. Connors to turn to Lizard. Also, Dr. Connors is a good guy, not a bad guy. He is not Lizard anymore. Lizard wants equal world, not revenge. Also, he is fully CGI, we don´t need this. Instead, when Sinister Six break prison and escape, then also Dr. Connors escape and he helps Spider-Man to defeat the bad guys. Dr. Connors and Peter are friends.
Spider-Man is not part of Avengers. Spider-Man back to Marvel? Hell No!