I like Lizard. He will be great in a Spidey movie. Even better than the Lizard will be the characters of Curt Connors and his tragically tormented wife.
I see the problem with a third villain spawned by "scientific experiment gone wrong," but I don't think the public is really keeping track of that kind of thing, and if they are, they aren't holding it against Marvel. To many non-comic fans, they just see a supernatural hero and a supernatural villain, and they don't even bother delving into origin issues. Most don't even distinguish between the stars of Marvel comics and the stars of DC comics or Indy comics. And that's to say nothing of the non-comic genre good guy vs. bad guy movies. So, as long as they've got Riddick and King Arthur and CINO and Potter and Shrek and Village and all the other various forms of good guys vs. bad guys, they aren't even thinking, "Oh great, another bad guy created by a scientific experiment gone wrong." There are lots of "supernatural" bad guys in lots of movies, Peter Parker just lives in a world where a lot of what happens relates to science.