Maybe they're going to do a Hobgoblin/Green Goblin thing. They could fool a lot of the fans by teasing Hobgoblin as the main villian, just to have him as a "copycat criminal" zipping around for a little while in the movie until Harry shows up in the Green Goblin costume and gets rid of him, before moving on to the main event with Spider-Man.
I hope it's not Sandman, that seems like a pretty weak alternative when they have several more interesting characters they could go to before him.
And this bit is from nearly a decade ago, but I remember Marvel was all hung up on doing a Venom movie independent of the Spider-Man films...of course, this was from back when the Spider-Man movie was still tied up in legal problems that prevented it from being made, so I'm sure a lot of plans have changed since then. I don't think Venom has enough support from comic fans, nor the recognition factor from the general public at this point to carry an entire movie.
"Come with me if you don't want paint on your clothes." was my favorite line, because I love a good Terminator joke. The folks at the AV Club comment peanut gallery are trying to dissect/list every possible action movie homage that was used: