I figured a good way to add to peter's pain, and give the symbiote an evil persona, and avoid the repeating theme of science accidents
pete works for conners on his experiments trying to help rats regrow limbs, but every time they successfully grow one, the rat transforms into like a rat lizard. He's been successful for the first time, and he is testing it on himself. so far everything is going according to plan.his arm is slowly growing back. in a small fridge conenrs has a batch on one side with the serum that had previously turned his lab rats into lizards. on the other side is the serum that works that conners is using on himself daily.
spidey goes to conners to ask him for some insight as to what the symbiote is. conners takes a sample to study it, and says we have to get this off of you as soon as possible. spider-man gives him a sample, of the symbiote so he can figure out a way to seperate him from the symiote. conners tells him he can go home for the night and come back the next day and he should be ready.
that very night while peter is asleep the symbiote possesses him and takes him for a midnight stroll(just like it used to in the comic book)
conners is at the lab studying the sampleof the symbiote. he is completely unaware that spider-man is in the lab. spider-man switches the serums. the symbiote makes him do this so conners will turn into the lizard and be incapable of helping spider-man seperate from the symbiote.
peter wakes up the next day, goes to conners lab completely unaware of what he did during the middle of the night.
then it becomes a race. spider-man has to cure conners so conners can cure him, before it's too late for the both of them.
that would be my spider-man 3. sandman and venom wouldn't be in the movie. but of course harry osborn would be green goblin. and I would have Ock back too.
that would also be a good way to work into eddie brock as a set up for part 4. eddie discovers what spider-man has done, but he prevents eddie from proving it, to protect conners and his family, making eddie look like a liar, ruining his career.