^^ I know several parents who wouldn't take their kids to Spider-Man 3 because they thought Venom would be too scary. If Carnage is in the next film, there's going to be even more parents like that...
In counter-argument, when I went to see Spider-Man 3 I saw more than one set of parents with toddlers/babies seeing the film. And no one walked out (or laughed or anything stupid).
I think Spider-Man 4 should have Conners become the Lizard and the symbiote sample should escape and our last shot of the film should be it slithering into Ravencroft. The Lizard should be the only villian in the film, though I'd also add in a prototype Spider-Slayer made by Jonah.
In Spider-Man 5 I'd have Jonah step the program up and fund the creation of the Scorprion (maybe even have John be the guy in the suit) and more advanced Spider-Slayers and, in the background, have Cleatus Kassidy escape Ravencroft in a big prisoner riot and stumble onto the sample. Our last shot would be a haunting shoadw image of them bonding.
Finally, Spider-Man 6 would be balls-to-the-wall Carnage style, with Carnage wreaking havoc throughout. Maybe he even trashes the last of the Spider-Slayers. If the film's not gonna show his true violence then it has to at least imply it loads. Carnage should be right out of the comics and brutal, vicious and violent. His downfall would come in the Boy's Home he was raised in. They could possibly bring Sandman back to team with Spider-Man in this one, just to show that he needs help.
I'd have the films be titled Spider-Man: [Something] rather than continuing the numbers system (especially because Hollywood avoids that now). The fourth film should be in the same sort of tone and vein as the first, the second should be bigger and more epic with loads of Spider-Slayer action and sympathetic insanity from Scorpion and the last should be dark violence, highlighting Carnage as Spider-Man's most insane and deadly foe. Somewhere over the course of the films they could kill off Captain Stacy and Gwen.
I see nothing to be gained from Kraven, Shocker or Chameleon being in the films. Kraven is pretty lame, as is Shocker, and Chameleon seems a bit crap too. If they want to put them in then they should be side-villains who have no baring on the plot (like Ultimate Shocker). Personally, I'd love to see Electro worked in too because he has the potential to be visually stunning and powerful (and is a better character than the Shocker). Mysterio could work, possibly, maybe even as a team up with Chameleon, maybe as a side-story trying to put bad press on Spider-Man.
Just for the record, Doctor Octopus, the Goblins and Eddie Brock are all dead. Dead. So I wouldn't see them returning, although it might be quite kewl to see John Jameson discover the Goblin equipment and become the Hobgoblin, but I think we've had enough Goblins for now.
Just my thoughts.