While I agree with a lot of what you are saying, if done properly, Venom could easily have been the ONLY villian.
We could watch black Spidey doing a downhill slide in the first act. He still stops bank robberies and such, but now does it in a more vicious, brutal way. He alienates the press, MJ, his aunt, the city, and even Brock, who could start out as an actual friend. Parker could move on to Gwen Stacy, who could be Brock's girl, and steal her from him. Parker could make Brock loose his job somehow, etc. Anyway, parker winds up screwing up his life and Brock's. Parker finaly realizes it, and gets rid of the symbiote, gets back with MJ, the city loves him again, etc. HOWEVER, Brock can't get his job back, and GS still has the hots for Parker, and tells Brock just that, blah blah blah.
Brock gets the symbiote, and at first, tries to be a hero like SPidey, in a desperate attempt to get both GS and others to like him. He could even start out with a fairly close copy of Spidey's black costume, and as Brock goes on his downhill slide, the costume mutates, responding to his worsening mental state. The webbing could go from nice and neat to mesy and random as his psychosis gets worse. Brock/Venom keep showing up at Spidey's actions, and even messes some of them up. Spidey tells Brock he doesn't want his help, etc. It pushes Brock over the edge, and he finally decides to battle Spidey.
Then we would see Parker realize what he could have become, as he sees the final version of Venom in front of him, a mindless, drooling idiot bent on destruction. It would also make Brock's death more of a sad, unavoidable end to his downward spiral. All the while, parker could be thinking that it could have been him instead of Brock.