I don't think there will be two Goblins as some folks have been speculating. My understanding is that Goblin will go through three costume changes—the original mask, hooded, goggles.
A redesigned Rhino would be my 1st choice for the 6th villain.
Wouldn't be surprised if it's Vulture, but to be honest I was not a huge fan of the suit. I actually thought the video game did a much better job of costume design for Rhino and Vulture. Scorpion was also very cool.
I don't know why folks are flipping out over Electro. I know everyone is happy to get rid of the blue lighting and translucent skin, but to me this new costume costume looks like a cobbled together mess instead of being sleek tech. I'm sure all those gadgets, baubles and packs are supposed to be functional power storage, dampeners, and what not, but that's no reason why it can't all be integrated into some kind of a cool, futuristic suit based on the comic design. Maybe there's an upgrade version that will come later. The electricity mask is cool in concept, but will have to see how it ends up being pulled off.
Marvel wants to sell toys, so we probably haven't seen the best versions of everyone.