I'm neither a Carnage hater, nor a Carnage lover. He hovers somewhere around #10-12 on my list of favorite Spidey villains. His story's flat, but he has some awesome abilities, so I thought to myself, "how can he possibly work in a film." and then it hit me.
Cleetus Cassidy, in his movie interpretation, would be a longtime hitman for Wilson Fisk, The Kingpin. Over the years of killing, Cassidy started to get sloppy and unprofessional. Instead of just killing the target, he'd kill their families, and he begin showing the signs of a complete mental breakdown. Kingpin used his connections to frame Cassidy as The Kingpin of crime.
That's right, Fisk would get EVERY SINGLE ILLEGAL thing he's been suspected of pinned on cassidy, resulting in multiple life sentences for Cassidy.
The Kingpin's bitter son, Richard Fisk, decides to use Cassidy's hatred of his father to his advantage. Richard decides to break Cassidy out. The cops pursue him to Empire State University, where Cassidy hides in a science lab. There he sees about a pound of goop, jumping around in a jar. He lifts the jar as the swat team breaks down the doors. The goop jumps on Cassidy and he goes through a painful transformation, and becomes Carnage.
Suddenly, all Cassidy can see is Fisk. Everyone likes like Wilson Fisk the Kingpin. No matter how many times he kills the Kingpin, there seems to be more of him...so he decides to go all out, and keep killing Fisk until he goes away.
The story would then unfold from the typical maximum carnage perspective...other elements would depend on when in the Spider-Man saga this took place, and what the Venom solo movie was like...but I think Fisk would be a good tie in with Carnage...It would allow Carnage to be crazy, without resting the plot completely on him; the truth of the plot would be Father vs. Son, The Kingpin vs. Richard Fisk.
The symbiote fanboys get a Venom/ Carnage action, the people who want to see plot get the Fisk family problems.
As for Peter, The Kingpin could have Captain Stacey offed (for being too much of a goody two shoes), and Carnage could kill Gwen, so Spidey would be stuck in this big war, with people he cares about dead...and stuck teaming with his ever lovin' pal Eddie Brock.
Like I said, I'm not a huge Carnage fan, but I always like a challenge, so I thought I'd slap this together.
This imo should be saved for AFTER the Lizard, and Kraven...but if they have to do Carnage, that's how I'd do him.