Kraven The Hunter is a much better compliment to THE LIZARD as the main villain story-wise. They don't have to adapt the entire Kraven's Last Hunt storyline, just use a few key elements. Like Kraven's obsession with Spidey and being his better by killing him.
If you want Carnage so much, why not wait until SM5 for him. Even though they can't even say the words SERIAL KILLER in these films, with them catering to kids so damn much. These films are too juvenile for Carnage to be rippin' into MoFos for the fun of it. No one wants to be looking at fade away shots for 2+ hours, trying not to scare children. You'd need an R-rated Spider-Man done like ZODIAC, SILENCE OF THE LAMBS or SE7EN. Just a warning: If you didn't like what they did to Venom, you would absolutely hate what they'd do to Carnage. He's 20 times the killing machine. His only purpose in life is to kill. I wouldn't mind seeing this in an R-rated form, anything else, no.
Not to mention, they already have a full movie with the symbiote, like it or not. The last thing they want to do is, give us another symbiote movie when they're trying take the film in a new direction. Think about that.