I looked everywhere for a thread for posting ideas for a fourth film, but I didn't find one. So, sorry for making a new thread.
I think that the best villains to use for a fourth movie would be The Lizard and Kraven the Hunter. The reason I think that is that it wouldn't take much time to introduce them. Dr. Curt Connors has already been in the second and third films. He's already an established character, and I don't believe it would take very long to get him from Connors to The Lizard. All that would be need to be shown is that he is experimenting, trying to find a way to regrow his arm after a couple of scenes with his family. He's at the lab late one night, and a tragic accident occurs, and he becomes The Lizard. He's wreaking havoc on NYC and of course this brings out Spider-Man. They have a big battle on the streets of New York and The Lizard takes Spider-Man to the limit. Eventually, The Lizard escapes into the sewers and Spider-Man loses him.
Of course, Jameson learns of this, and does his usual gig. He criticizes Spider-Man in the Daily Bugle for not being able to get the job done. Jameson, trying to make himself look good, somehow gets in touch with Kraven the Hunter and brings him to New York City, claiming that Kraven will do what Spider-Man could not do, which is capture The Lizard. Once again, Kraven doesn't really need much of an introduction, and it's probably better that way anyways. He's known as the world's greatest hunter, and that should be about all that needs to be known.
Peter is losing confidence in himself. He couldn't take down The Lizard, and things just keep getting worse. Aunt May is very ill, and doctors are not giving her long to live. This starts Peter's story for the movie, which is him being upset about how he can do all of these amazing things, and has saved so many people, but he cannot save Aunt May, who has been so wonderful and caring to him.
Meanwhile, Kraven the Hunter is prowling the city, seeking out The Lizard. He's having a difficult time. Eventually, he does find The Lizard, but is unable to capture him. After a couple of failed attempts, Kraven is starting to get frustrated. This is his toughest hunt yet.
About halfway through the film, Peter has a tender moment where he's with Aunt May in the hospital, and tells her of his disappointment about not being able to help her. She tells him "You cannot save everyone Peter. Some things are just out of your hands." Peter doesn't respond to this, he's in shock. Does she know that he's Spider-Man?
The next morning, Aunt May passes away, and Peter is devastated. Kraven is tearing up the city trying to capture The Lizard, but Peter is just not able to go out and try to stop them. He's still having issues with his confidence. Then, Peter sees a report on the news about Dr. Connors missing. There is an interview with Dr. Connors wife, where she tells the press that the last she had seen him, he had been working tirelessly on trying to find a way to regrow his arm. The young child, Billy, is so lost without his father, and Peter knows what this is like, having lost Uncle Ben.
This wakes Peter up, and it makes him determined. He has put two and two together. Dr. Connors is The Lizard, and Peter is the only one who can save him. Peter gets to work on a way to change Connors back to his normal self. Using his scientific skills, he finally concocts a formula that will hopefully bring Connors back. This brings us to the climax of the film.
It's early morning, and the sun has just started to come up. Kraven finds The Lizard, and promises to himself that this is the last stand. The Lizard has made his way to the top of the Empire State Building. Kraven follows, and a huge battle ensues. Kraven is giving it his all, but The Lizard just isn't going down. Spider-Man shows up, and tries to inject Dr. Connors with the cure, but is unable to get him to stand still enough. The Lizard throws Spider-Man off and makes his way to the sewers. Kraven tries to follow, but Spider-Man stops him, telling him that there's human being behind that monster. Kraven doesn't listen, and continues after The Lizard. Spider-Man refuses to allow this to happen, and a the big Spider-Man Vs. Kraven showdown is on.
Kraven is much stronger than Spider-Man, and takes him to absolute limit. Kraven has Spider-Man down on the ground, and tells him "You're in over your head, Spidey". Peter knows that he can do this, he can save Dr. Connors. He finds a second wind, and somehow defeats Kraven, knocking him out and webbing him up. Now it's time to save Dr. Connors. He walks over the ledge and drops off, heading towards the sewers.
He searches everywhere, but cannot find The Lizard. Then, finally, Spider-Man feels a some very heavy hot breath in his neck. The Lizard smacks Spider-Man with his strong tail, sending him flying. This is in a big, open area of the sewer, so the battle is huge and exciting. Spider-Man is still determined, and almost holding his own, until The Lizard gets him up against a wall. The Lizard is about to chomp Spider-Man's head off with those long sharp teeth, when Spider-Man shoots a web onto The Lizards mouth, holding his jaws together in a muzzle. Spider-Man knows this won't hold long, and takes one last desperate shot with the cure, and gets it.
Dr. Connors becomes his normal self again, but passes out. Spider-Man carries him to the surface, and takes him back home. He explains what happened to Connors wife, and when Connors wakes up, Spider-Man tells him he should never try something so dangerous again, for he has a family who loves him, and never want to lose him. He tells him he can save his future, and to never do this again. Connors has come around, and realizes the error of his ways. They are a happy family again, even is he does only have one arm.
The movie ends with Peter at Aunt May's grave. He stands there, though, knowing that Aunt May would be proud of him. He webs a heart onto her headstone. There is a voiceover from Peter. He says he may not have been able to save Aunt May, but she was right. It was out his hands. There ARE things that cannot do, things he cannot stop. All he can do is do his best, and make the best out of what he CAN do. We fade out. The end.