But, I have an idea.
Spiderman 3 deals with the Venom/Carnage encounter. In the beginning, something happens and Spiderman has to hide. This is where Venom comes into play (in the cartoon, the Venom symbiote could morph into suits of clothing, which would be a really cool effect on screen). He uses the black suit to continue his work.
But, the symbiote has started inadvertantly accelerating the DNA changes. All throughout the picture, we get hints of the changes, physically. Soon, he realizes what's going on, and he discards the suit, and it splits, and the 2 symbiotes find new hosts.
In the end, he defeats Venom/Carnage. Part of me says he should transform into a full-blown Man Spider at the very end, and that is how he defeats both. The other part of me says that's too much of a cliffhanger, and if a 4th movie isn't made, it too much to leave undone.
Then, if/when they do Spiderman 4, then we have the full-blown Man Spider, and the attempts to have him change back (intro the Lizard/1-armed doctor that can help him when he's not transformed himself, add Scorpion as JJJ's attempt to destroy Spiderman).