Spider-Man's rogue's gallery is so extensive that it seems strange they would pick Moribus over some of his other enemies, like the Lizard or Kraven the Hunter. But, if they do, it would certainly be a cool movie.
I like the idea of Michael Morbius being a friend of Peter Parker, working together at ESU, until a lab accident changes Morbius into the living vampire, and Spider-Man is the only one who can stop him. Even though the "friends-become-enemies" thing was done with Peter and Harry in Spider-Man 3, I feel that Sam Raimi can add much more depth to this and make it feel original.
I just hope that the reason they are putting a vampire into the movie isn't to play on the popularity of the Twilight series. As I despire Twilight and all of it's sequels, I don't think Spider-Man, as a property, needs to tap into this market. Like I stated earlier, Spider-Man has so many enemies, so if they are only trying to play up to the current popularity of vampires, then you can count me out. I would rather them pit Spider-Man against the Spot (which, come to think of it, would be pretty cool.)
Although, I would love to see Spider-Man kick the shit out of Edward Cullen...