Thats what I thought when I was told about this.
I can't imagine who he would be though..
As far as any rumors that i've heard about we know we're getting some sort of harry goblin.. i'm not sure if he'll be a green or hob.. who knows how they will reinterpret things..
But there are major rumors and statments about the Sandman. Mostly of a lot of concept art for him and such. Sandman isn't my favorite, but is still cool. Although he probably won't look too real. but who knows with the advancements in effects..
Aside from the goblin.. I'd almost expect the Lizard to be in it. Doc Connors was mentioned in the first movie, and even had a bit role in the second film, so I see it as very likely.
I'd like to see the Vulture, but considering the baddies we already saw spiderman go up against, i think he isn't tough enough.
Electro and Mysterio would be pretty cool.
Scorpion might be cool too, but he's basically a lesser doc ock..to me anyway..
Kraven.. i dunno..
It would be cool if they did a Sinister Six thing.. but that'd have to be it's own movie.
I don't know why everyone wants Venom and Carnage to team up on spidey.. as I recall it they are enemies.. and Venom has helped spidey to defeat Carnage before..