Venom isn't confirmed. It's all speculation based on a comment from Kirsten Dunst. But remember, she let slip in an interview for SpiderMan-2 that the enemies would be Doc Ock, The Lizard, and BlackCat yet the other two never appeared.
Venom isn't the classic spidey villain. The more likely scenario is that Topher Grace would be Electro or Lizard. The Lizard actually fits because Spidey's enemies in the movies have been scientists like Peter who unfortunately were turned into super enemies by an experiment gone wrong, but who may have been nice guys otherwise.
What would Venom's origin be? No way it would be an alien symbiote suit. If Venom is in the movie, he will be tuned WAY DOWN from the McFarlane vision. He'd be a wimpy teen like Peter. Somehow he would be exposed to Spidey's DNA, perhaps he runs into Peter running an experiment on himself. He will gain Spidey's powers and make his own Spidey suit. I think he would bear very little resemblance to the Venom of the comics. Venom is too stupid and thoughless of a villain for Spidey.
Sandman's origin will be re-imagined. It won't be getting zapped with nuclear radiation on a beach. And it won't be some DNA mutation because DNA mutation won't explain the "sand"
Rather, I think it will have something to do with nanotechnology. He will run into a government or military nanotechnology experiment based on silicon nanites. He will be "infected" by them, and come to be able to control them.