Let me try this...
What if the movies had gone this way...
Introduce Brock in SMII. He works with Parker at the Bugle, and they actually get along pretty good. Parker even helps him a little by letting him get some pics of Spidey when he needs extra cash, maybe introduces Brock to Gwen, etc.
As Harry pulls more and more away from Peter, because he resents Peter's apparent friendship with Spidey, Brock and Peter actually get closer as friends, as Peter confides to Brock that his knowledge of Spiderman is ruining his friendship with Harry. Peter and Brock go out for a beer (or I guess a soda, since I don't think they would be 21 in SMII) and they both pledge to not let Spiderman come between their friendship. Pretty hokey, but the point is, Brock and Peter get closer as Peter and Harry grow apart
Now SMIII. Parker gets the symbiote (Strange, Microsoft Office does not recognize symbiote as a word. Am I spelling it wrong?) very early in the film. First ten minutes. We see Parker start his downhill slide, getting more brutal and violent both as Spiderman and as Parker. Brock starts to pull away from Peter just like Harry did; only the symbiote influenced Peter really doesnt care. As a matter of fact, Parker starts to sabotage Brocks work, and starts to steal his girl, Gwen. Brocks world starts to fall apart, and he blames it on Parker.
Now, Parker has whatever epiphany he has to get the symbiote off him. Cut to the bell tower scene. Brock is there not to pray for Parkers death, but just for help and guidance. He still sees Parker getting rid of the symbiote, and it still attaches to him, etc.
Brock then tries to become a hero like Parker, using the powers the symbiote gives him. He shows up on Spidermans actions, and causes more harm than help, maybe even getting innocents hurt a couple of times. For all intents and purposes, the costume looks pretty much like the black Spiderman costume, maybe with the purple hue to it. Brock tries to rekindle his friendship with Parker, telling him his life is better now that he has the symbiote. Parker tries to tell him to get rid of it, only bad things will come of it.. Brock, showing the early influence of the symbiote, says that Parker is jealous because he (Brock as Venom) is stronger than Parker as Spiderman. Brocks costume changes as he slips into insanity. The normal, neat webbing starts to get ragged and random, the eyes get bigger and more angled, etc. as the symbiote feeds of his deteriorating mental condition.
Meanwhile, Brock had started to get his life together, but his worsening condition causes him to fall out of favor with the public, etc. He tries one more time to rekindle his romance with Stacey, but she rebukes him, telling him she still has feelings for Parker. (Parker has since went back to MJ, who of course forgives him. She is such an enabler.)
Gwens final word on their relationship is the straw that breaks the camels back for Brock. I could even see Brock making a very clumsy (and very PG rated) attempt to force Gwen to kiss him, and this could be the first time we see the Venom tongue
So Brock leaves Gwens place, and goes to Parkers, to one last time try to patch things up. When he gets there, Parker and MJ have obviously just finished a romantic interlude (again, very PG rated). MJ, not realizing what Brock is going through, blurts out that Parker has just proposed, and shows him the ring. With that, Brock looses it, and begins a mental break down in Parkers apartment. He starts pacing back and forth, ranting about how Peter and Spiderman have cost him everything. He starts to froth at the mouth, with little tendrils of the symbiote creeping up his neck, but he doesnt notice. Parker and MJ try to tell him to calm down, as they see what Brock cant. Finally, The symbiote fully takes over, and the throwdown begins. Venom just rants about how Parker and Spiderman have ruined his life, and the epic final battle begins.
Now as a subplot, Harry has been after Parker during the whole movie, with the current GG2 storyline, and his use of the serum, etc. Harry sees his chance to finish Parker off, and joins Venom in the fight. Spidey takes one hell of a butt whipping, but at several points in the fight, has the opportunity to kill Harry, but never does, saying he would never kill his friend. It comes down to Venom about to about to finish Spiderman off, and Harry realizes the error of his ways, and stops the deathblow. Venom lashes out and kills Harry. Spidey takes the opportunity to pin Venom down, and to try to talk some sense into Brock Brock wont have any of it, and continues to try to kill Spiderman. Spiderman finally gets the symbiote off Brock, but the drain on Brocks body is too much, and he dies. Brock, however, never sees the error of his ways, and his last words to Parker are that he still hates Parker
Parker then rushes to Harrys side, thanking him, etc. Harry dies shortly after making peace with Parker. Parker tells Harry that he is giving up being Spiderman, as it has ruined so much in his life. Harry tells him not to, as that would make it all for nothing
This would bring in the redemption of Harry, and show that some people cant be redeemed
Anyway, that is how I think Venom could be the main villain, AND keep Harry in the story, but it would have had to start with SMII.