I agree on Venom being the best, but the earliest it could happen would be a four or five if it was going to be done properly. As far as I’ve heard, Sam Raimi wants to keep directing the movies as long as he can make them, so Venom would be set up as opposed to dumped in.
On the IMDB trivia page for Spider man 2 it says that the villains for 3 will be lizard and Green Goblin. I know, everybody hates the Goblin. Just think of how effective it will be to have Spidey assaulted by the goblin though. Remember, he thinks that the Green Goblin (Norman Osborne) is dead. He won’t know that it is Harry in the suit. He’ll just see the costume and hear the voice (synthesized of course) and freak out. Throw in the Lizard (Peter Parker friend/father figure/all around nice guy Doc Connors) and you’ve got the right mix of angst and action.
At this point, can’t we all agree to have some faith in Raimi and crew. They haven’t let most of us down yet and likely will not with the third installment.
As for the Black Cat, I would love to see her too. She was the one love interest that could be there for both Peter Parker and Spider Man, so it could really throw a wrench in the Peter/MJ thing. I just don’t see that happening until later.