They could easily introduce Brock in 3 if they made him the son of Peters Dads partner, ie the Ultimate storyline, but i still dont think they would do that.
The most important thing from the movies point of view is to wrap up the trilogy (I know Wilco is saying Rami might to 4, but if the cast doesnt want to, the arc still has to end at 3) , not give lip-service to the fans by tacking on another bad guy who doesnt fit with what needs to be done, namely resolving Harry Vs Peter. Venom is way to big a story to fit in a film that is about Osborns decent into madness and revenge on Spider-Man.
The most important thing from the movies point of view is to wrap up the trilogy (I know Wilco is saying Rami might to 4, but if the cast doesnt want to, the arc still has to end at 3) , not give lip-service to the fans by tacking on another bad guy who doesnt fit with what needs to be done, namely resolving Harry Vs Peter. Venom is way to big a story to fit in a film that is about Osborns decent into madness and revenge on Spider-Man.