They won't have a cliff-hanger in my opinion. Harry is like almost confirmed as GG2 now go to villain thread and click on link. That almost guarentees that Venom won't be in it. Too much, in too little time. So even if Brock is introduced in 3 I don't think we will see the symbiote though. Maybe that in 4, and transforms in 5. Spiderman 4 - I can picture it now, Lizard and Scorpion! The poster shows Jameson behind Scorpion and Conners behind Lizard. With Spiderman/Peter looking scared as crap. lol
If they don't have venom they should at least have spidey having to use the costume to beat both villains. Like he finds it hard to beat them both and then he finds the costume then he is able to beat them, finds outs costume is alive and rejects it, 4th film starts with Eddie moaning about spidey and has costume attatch to Eddie.