I'd love to say Venom, but alas, the Venom storyline is too big to fit into a single movie.
However, it is possible that the black suit may be intoduced in Spiderman 3, which could then be followed by a Venom plotline in Spiderman 4 (and maybe even Carnage in Spiderman 5)
Now, i'm sure my fellow comic nerds will recall that Spiderman first encountered the black suit symbiot on an alien planet during the Marvel Comics Secret Wars, and that storyline is so involved that it would be impossible to work into these films.
BUT, in the cartoon series, the astronaut John Jamison inadvertantly brings the black suit symbiot back from one of his space missions, there's a problem landing his space shuttle, and Spiderman rescues Jamison & encopunters the symbiot & gets the black suit that way.
And THAT, my friends, IS easily acheivable on film. Especially since the John Jamison Character has already been introduced. All they would need to do is introduce Eddie Brock in Spiderman 3, just enough so we'd know who he is in part 4