The rumored villains have been Electro, Green Goblin and Doc Ock (now all confirmed) and as for the rest... Sandman from the Maguire verse, Lizard from the Garfield verse and both Vulture and Mysterio from the Holland verse (or a variant of both) have been rumored. So six of of those seven. One of the advantages of Sandman, Lizard, Vulture and Mysterio is that they don't have to show the actors faces in theory. The trailer does appear to have a sequence with a combined Electro-Sandman attack.

Grace Randolph also has an interesting theory (her own) that Stephen Strange ISN'T our Strange but either the evil Strange from the What If series (remember Feige said some characters could crossover to live action) or the popular choice of Mephisto.