Actually, the more I think about it, the more I think they got it right. Consider the quote at the beginning of the Comic Con trailer from Aunt May - about how seeking revenge turns you into a monster. They're not basing this film around a single plot strand, but around a theme.
Peter/Spidey will seek revenge on Sandman because he is led to believe Sandman was involved with the killing of Ben.
Harry seeks revenge on Peter/Spidey because of his father.
Brock seeks revenge on Peter for whatever reason is revealed in the film.
So essentially, the film charts the corruption that revenge has on each of these characters. And revenge warps and destroys each of them in their own way, turning them into monsters. In Brock's case, this happens in the most extreme form, turning him into a slathering, horrifying creature.
That is why I think they've left Venom's appearance to the end. Because it's the most extreme, the most shocking, the most terrifying realisation of Revenge.
And once again they're clearly focusing on character - which is why Raimi said that each villain is brought in tightly connected with the story. There are no superfluous conflicts in other words - and that adds up to one hell of a suspense-filled film.
I've had my doubts about Venom and all the other villains being crowded in, but the more I see, the more convinced I am that there's nothing to worry about...
*deep breath*