Title says it all--I have a few ideas of my own. SPOILERS for ASM 2, be warned.
Scorpion - keeping in line with villains we haven't seen onscreen yet, for me Scorpion is a must. I'm not sure what his motivation would be as a character. But I would love to see that suit onscreen, and not all overly mechanical where you see every nut and bolt like Transformers or something. Keep it simple, streamlined, green, with a tail that smashes **** and shoots lasers, as well as pincer claw hands for scaling buildings and chasing Spider-man.
Shocker - I would love to see Shocker in a Spider-man film, even if he's just in the beginning of a film robbing a bank and gets dispatched by Spidey. But he has to be in the full quilt costume (which would get a lot of jokes out of Spider-man). I would just really dig seeing him throw punches across a room with his vibro-gauntlets that Spidey has to avoid with speed--could be a great action sequence.
Venom / Carnage - as they've already done the black costume thing in Raimi's SM3, there's really no need to do it again. So that means they may as well forego Venom and go straight to Carnage. I guess it would be another experimental suit that Richard Parker was working on thing like in the Ultimate Comics. And since they've already introduced Ravencroft, it seems logical to go straight to Cassady / Carnage. It would also be in line with a darker take on the movies post-Gwen's death, if that's the route they intend to go.
Mysterio - well, duh, he's one of the all-time great Spidey villains. I'm just not sure yet how he would work onscreen. But they may as well do the Ultimate-styled look perhaps mixed with the JR Jr. redesign of the 90's.
Having said all that, the only villain I would love to see is Hobgoblin but they would probably never do it because it's a character that works so much better in legacy form in the comics. Hence why we'll never probably get a good Hobgoblin portrayal on film.