My idea
Spider-Man 4 - Man-Spider
A piece of the symbiote at the construction site lives on and carries with it enough genetic information to reconstitute itself and Brock. Venom returns.
Meanwhile an old opponent of Harry's father, Silvermane, orders a local gang led by Roderick Kingsley (Dan, Montana and Ox in tow) to raid the estate of the late Harry Osbourne. Spider-Man however, is there to stop them. Silvermane tells his associates to detonate the, and all of them escape the explosion (Spider-Man as well) with the exception of Roderick who takes refuge in the testing lab. Underneath the rubble, Kingsley is exposed to the fumes becoming like the Green Goblins before him.
Mary Jane eventually landed acting work in between the films, as a "Hula Girl" on a reality TV show chronically the exploits of Kraven the Hunter, played by Bruce Campbell. Kraven is chauvanistic, and very much like Ash. In addition he has a thing for "hooking up" with his Hula Girls, especially the engaged Mary Jane.
Kingsley frees himself from the wreckage and salvages what he can from the glider, bombs and equipment. Hiding out in an abandoned playhouse he steals several items dawning a costume of a Goblin. Using his newfound strength and power he kills Silvermane and usurps his position as the head of the Enforcers.
Venom reborn, now with the lines between Brock and the symbiote almost extinguished, finds out that Dr.Curt Connors has been working on a piece of the symbiote, and has published several articles about a retro-virus that could potentially re-grow limbs. At the time however the effects are found to be cancerous.
Kingsley attempts a theft of the Daily Bugle to coax the Wallcrawler out into the open. They fight, and sadly for Kingsley, he fails miserably, severely over-estimating his own powers and abilities. He does however manage to get away, while the Enforcers do not.
On the rooftop that night Venom finds Kingsley. Venom asks what Kingsley name is, and he now names himself Hobgoblin. Venom says he has a plan to end Parkers miserable life by making his own Spider-powers work against him.
Kingsley, following Brock's plan, steals a sample of the retrovirus from Connors. In great villain fashion, Hobgoblin even tells the helpless Connors his (their) plan.
Kingsley stages yet another robbery, of a bank, this time with Brock posing as a hired thug. When Spider-Man gets there, much to his chagrin, Venom reveals himself. The two fight, and, admidst the confusion Hobgoblin shoots Spider-Man with the retrovirus. Spider-Man immediately lashes out in a rage at Venom, but not too long following he calms down and passes out allowing Venom and Hobgoblin to escape with the money. The Daily Bugle has a field day with the story.
Peter wakes up back at home with a visiting Doctor looking after him. The Doctor says his cold appears to be fine and leaves. Mary Jane said he had been out for two days. Peter says he is feeling fine, actually better than usual.
Later Peter is covering the making of the Spider-Man movie. Ben Reilly is in the starring role. He is a total jackass. Peter ends up in conversation with some annoyed fanboy in the crowd.
Across town Mary Jane is looking over Wedding dresses with Aunt May. Aunt May is giving her motherly advice about how she must stick with Peter even through the troubled times. Marriage is about committment.
Peter begins to lose control of his powers slowly. It first appears in Curt Connors lab when he cannot stop sticking to various equipment. Over the course of interactions with J.Jonah.Jameson, Mary Jane and even Aunt May Peter appears to becoming more moody, less intelligent, severely less intelligent, more primal, he starts to notice hair growing on his chest and so forth.
Finally his temperment, poor hygene, and slipping grades alert the attention of Curt Connors, who asks him to come by after class. Peter does. Peter has a debate with himself outside the office but finally resolves to tell Connors he is Spider-Man and what (he thinks) has happened. Connors immediately knows what has happened and tells Peter he needs to stay in his lab until he can find an antidote. His condition severely worsens overnight, growing eight eyes and hair all over his body. Peter tells Connors not to inform Mary Jane or Aunt May about his whereabouts to spare them the pain of seeing him like this.
Connors works furiously on the antidote, but one night in his lab the Man-Spider is born. He breaks out and immediately seeks out the residence of Aunt May. He breaks into her apartment, waking her from her sleep. He manages to slur her name, but she cannot bear the sight of this creature and suffers from Cardiac arrest. Mary Jane is called on set of Kraven and is told to come to the Hospital. Aunt May is alive, but barely.
Connors is waiting for Mary Jane at the Hospital and he informs her on what has happened. Connors tells her he in fact has created what he thinks is an antidote, just too late to stop this.
Connors brings Mary Jane back to the lab, he shows her the antidote. He asks her if she knows someone who could track down Peter and administer it to him. Begrudgingly she says yes.
Man-Spider is found roaming the sewer system by Venom and Hobgoblin. Venom feeds Man-Spider several mice until he is satiated, and then they lead him to the surface. In effort to get him to destroy everything he holds dear they unleash him on the set of Spider-Man:the Movie. He begins the raise caine, thrashing and destroying set pieces. Ben Reilly flees the scene.
Meanwhile Mary Jane and Dr. Curt Connors attempt to sway Kraven the Hunter to help them. After much bickering, Mary Jane tells Kraven he is a fraud, to save his ego Kraven agrees to help.
Kraven brings his dart gun and fills the darts with the antidote and Curt Connors lab. Curt Connors insists that he come along, but Mary Jane doesn't want him to get hurt and she and Kraven tell him to stay behind. As they leave Curt Connors spies a Iguana in a cage across the room and he looks down at the retrovirus he is holding in his hand.
Kraven and MJ get to the movie set where they find Peter has cacooned most of the staff and people present, including Ben Reilly who won't shut up. Venom and Hobgoblin, not feeling very threatened by the two, decide to let Man-Spider take care of them. The two "fight" Man-Spider, unfortunately Kraven is unable to hit Man-Spider with a dart, getting down to his last two before having the gun taken from him. Mary Jane picks up the gun and successfully fires the antidote into Peter. He changes back and passes out. Venom and Hobgoblin come down to their aid. Just then Curt Connors appears. He injects something into his arm. Nothing happens.
Venom throws Curt Connors into a pile of rubble and buries him there. He turns his attention back towards Kraven and MJ. Kraven tries to run but Hobgoblin stops him. Just then a giant Lizard creature wearing Connors lab coat comes out of the rubble and attacks Venom. He is easily equal to his strength and fighting prowess.
Meanwhile, while Hobgoblin and Venom fight the Lizard, MJ tries to wake Peter. Eventually she does and hands him a Spider-Man costume. Spider-Man swings into battle. He takes out the Hobgoblin first by knocking him unconscious. Parker then gets to the control panel for the sound system. He then lures Lizard and Venom next to the speakers. Spider-Man wisks Lizard away and then gives MJ and Kraven the sign to crank it. The sound blasts Venom, he begins to lose control of his form and is reduced to a puddle of black goo.
They get Lizard to a nearby alleyway but he starts to attack Spider-Man. Spider-Man takes off his mask and shows him the face of Peter Parker. Lizard weakly says Parkers name then passes out; Kraven shot him in the back with the last dart. Connors starts to revert back.
The cast and crew are freed. Reporters swarm the site. Kraven is heralded as a hero by the press. However in mid interview Kraven relents and tells them that it was in fact Mary Jane who stopped the Man-Spider.
One Month Later: Peter and MJ go to visit Connors at his home. He is their wife his wife and kids. He thanks Peter for saving his life, and Peter thanks Connors for saving his own. Connors says he has destroyed the retrovirus after seeing it's capabilities. Peter and MJ leave. Curt Connors goes down to his basement revealling a whole labratory set up to recreate the retrovirus.
We see MJ getting ready for the wedding, getting her dress on.
Peter meanwhile is visiting Rikers Prison. He walks into the Maximum Security portion using his press pass. There, across from each other are Roderick Kingsley and Venom. Roderick is in a typical cell while Venom is in a plastic cell with no doors or windows. Peter asks them when theyre going to reveal his identity to the world. Kingsley tells him he isn't going to, he'd rather see him squirm. The two villains start laughing as Peter leaves.
The last scene shows Peter and Mary Jane getting married.