Let's face it, Iron Man's doesn't have the greatest rouges gallery. In fact that's a bit of an understatement. The Mandarin is generally considered to be his greatest foe and most fans figured after the hints in the previous movies, we would see him in a threequel. But new director Shane Black seemed to dismiss this by calling the character a "racist caricature". Of course that doesn't mean he's not a possibility, maybe just a tweaked version will appear. Black did hint early on that this movie would be more tech/terrorist based. This certainly sounds like it could mean The Ghost, a saboteur with equipment that allows him to become intangible. Then of course we have that Namor rumor. It was a stretch (the studio where the movie is being filmed will employ a special effects water tank) to begin with but Namor has faced Iron Man a few times in the comics and we know Marvel have at least thought about the character because of those Easter Eggs in Iron Man 2.
So what about some other villains that haven't been suggested or hinted at? Well there's the Alien Dragon Fin Fang Foom..a highly unlikely inclusion but one that many fans would like to see, and would tie in nicely to any cosmic ideas Marvel have for future movies. They might decide to go down the "son/daughter out for revenge" route. Both Justin Hammer and Obediah Stane (Iron Monger) have spawned a couple of angry kids in Ezekiel Stane and the Crimson Cowl. The latter would also be a first for Marvel in utilizing a female villain, as would Madame Masque who was also a lover of Stark's for a time. There are plenty more possibilities of course, if you think it will be someone else or perhaps an amalgamation of two or more characters choose "other" and specify in the comments section. And remember, you are voting on who you think the villain will be, not who you would like it to be.