Vulture

This is the most well-known rumor: Michael Keaton will portray the Vulture in the first MCU Spider-Man flick. Just as Kevin Feige said, the first Spider-Man will feature a villain unused until now.

The rumor is that Keaton's character will have developed an exosuit similar to Iron Man’s. However, since he is not a genius like Tony Stark, the suit feeds on the wearer's vital energies instead of some arc reactor. Basically, once it is worn, it cannot be taken off. Pretty much like in Clown.

Keaton's character's nickname is the Vulture because of his ability to reproduce and reverse-engineer any piece of technology. Which is cool, I believe, and makes the character a multi-faceted one. What is more interesting is that the Vulture will win every confrontation with the web-crawler until the final one.

And then he will be basically outsmarted by Peter Parker. Which is yet again very cool, showing that Parker is himself a genius.

But there is a twist concerning the Vulture. He is not actually a bad guy before putting on the suit. The bad guy is someone else.