Even though I was unhappy with how Spider-man 3 turned out it was still a pretty good movie. When I heard they were making a 4th and 5th I was a little skeptical with the way comic movies have been changing the stories of beloved characters (the hottest one being Deadpool in the up coming Wolverine Movie). I have read many of the articles on this topic and had some ideas pop in my head though when it comes to the next spider-man movie and here they are.
The Venom symbiote got a bad wrap in the last movie, technically not all of it was destroyed. Remember, Peter took a sample of it to Doctor Conner (For those who don't know, he later becomes the Lizard). So, if they wanted to they could bring in Carnage from that angle and make it work. I also noticed that they introduced Topher Grace as Eddie Brock jr, and that he was a photographer. Now, in the comics it was just Eddie Brock, and he was a reporter NOT a photographer. Once again, technically, if they wanted another shot at the Venom Character they could introduce Eddie Brock sr, distraught over his sons death and that being compounded by his being fired from the bugle for fabricating a story, becomes Venom. It's a stretch, but if done right it could be an interesting story line.
On the Carnage front, I could see them taking some story line elements from the ultimate marvel universe, and have Carnage be the one to kill off Gwen Stacy and not the Green Goblin (Although I think many Comic fans would be upset at this).
I think they should finally introduce The Lizard. They have been working that character up for the last three films, it would be nice to finally see him. And if they wanted to go with the symbiote spin they could have dr. conners doing research on the symbiote sample from Peter, testing it's regenerative properties and some how work that into becoming the Lizard (once again taking some elements from the Ultimate Universe, because the venom symbiote was used as a protoplasmic dip designed as a cure for cancer). Then they can introduce Venom or Carnage from the symbiote sample.Cletus Kasady could be trying to hide after just murdering a student and stumble on the symbiote, or reintroduce Venom, maybe the symbiote escapes and goes looking for Eddie sr after sensing the death of his son. I can see the potential for a pretty good story here, as well as giving a darker tone to the spider-man franchise.