I think Chameleon, Shocker, Vulture and or Scorpion (maybe also Kraven) would all be fairly easy to set up, so one or two of them would probably make sense when rounding out the roster following Electro, Lizard and Rhino. I think it's almost a certainty you'd get Green Goblin as the next film's "big bad". I'd like to see Shocker and Scorpion; both make good henchmen types, and consider the current film's villains they require minimal explanation. Shocker could be a "Iron Man" tech type character, someone audiences are all reasonably familiar with. All you really need to do is introduce his tech weapons. Scorpion fits the "animal villain mutants" we've already seen with the Lizard and probably will see again with Rhino, so you already have the set up there between Parker and Connors. It's a logical extension of those two characters already introduced.