I personally think that they should have kept Harry as one of the villains, and maybe develop the Sandman storyline and character a little more.
They should have spent more time developing a little more with Harry. I felt like out of nowhere Harry tries to kill Peter while on his Moped. I'm like "huh"?? I mean, if they had more dialogue together other than that night they saw each other at MJ's play, it would have been just a little more believable. What happened to Oscorp/Harry after part 2?? What does Harry now do for a living? I mean, not that he NEEDS the money or anything. lol
Also, I think that it would have been fine to introduce Eddie Brock's character, but I wish they developed him more.
I would have been fine with them developing Eddie's character more, and perhaps Gwen Stacey and their relationship. And THEN saving the Venom/symbiote story for part 4. Personally...that's just me though.