I know we've probably seen way too many Spider-Man scenes theories and/or videos - And I know that most of us don't want Tobey Maguire back as Spider-Man (I am one of them. He was good, did a great job, but I want someone new) but just hold on...
The ending scene to every Spider-Man movie made by Sam Raimi ended off with a "final swing" (if you will) except for Spider-Man 3 - the movie that ended the trilogy. Spider-Man 3 ended...
... with Harry Osborn, Peter's best friend, dead and Peter and Mary Jane dancing in a room, not knowing what will happen next.
My theory that could fit into the Marvel Cinematic Universe is that Peter retired from being Spider-Man and decided to settle down with Mary Jane and become a scientist/professor at Empire State University.
Now that I've set the scene, let's cut to the chase...
A page turns in a notebook. A familiar voice to Spider-Man fans is heard giving a lecture on survival of the fittest (reference to Civil War) to a class in the background as the student who turned the notebook page continues to take notes.
The camera zooms out from the student taking notes to the entire class, then pans to the front of the room where we see the man giving the lecture with his back turned writing on the chalk board. The man stops writing on the board and turns around to finish his lecture, revealing the man to be Tobey Maguire as Peter Parker.
"... and that's where we'll pick up next time. Have a good day everyone!"
Peter begins to pack up his briefcase as the class makes their way for the door. Peter then turns his back again to erase the board.
"That's a pretty interesting lecture you gave there. I really like how enthusiastic you are. It draws me in."
Peter Parker turns to Stark, surprised.
(Shocked) "You... you're Tony Stark!"
"I am... and you are..."
"A huge fan! I've read about your work. It's amazing - the things that you've done."
"Not what I was going to say, but thanks for the complement... I was going to say that you're just the man I wanted to see."
"Well, what can I do for you?"
"I think you already know..."
Peter's expression goes from ecstatic to concerned/hesitant.
"I don't know what you..."
"Yes you do. Come on. It's okay. You're secret's safe with me..."
Peter ponders for a moment.
"... What do you need?"
"Another ally... The world needs you back in the game, Parker... the world needs Spider-Man..."
Peter is shown staring at Stark with a shy look, but you can't help but feel how heroic Peter really is underneath.
Camera cuts to black.
I hope you all enjoyed my custom post-credits scene.
I realize it won't be happening since Marvel is looking to bring Spider-Man back to high school, but I just thought that would be a cool scene.
Have a good one!