They COULD do venom, but I don't think carnage would be a good idea. Like you said, the fact that he's fucking insane wouldn't be shown very well, especially by the fucking dork they cast as him (bubble boy...ugh) and the cgi would be a bitch. Venom though...Venom is very possible but it would take at least 2 movies to tell a good story about the symbiotes and him.
Bah. The thing is, there's a ton of good spider-man stories they could do but they'll probably just do what they did for the first which is take a character, fuck around with him a lot and just put him in the movie.
Anyway, I've got a good amount of episodes from the cartoon downloaded so I'm happy for now (secret wars, all chapters. Final 2 episodes and more
).