And what I'm saying is it takes more then a villain with a gimmick to make for a suitable Big Bad. Every villain has a gimmick, but a gimmick is not a character and a lot of comic villains only have a gimmick rather then a defined personality.
Spot has a gimmick, but he makes for a lame super villain, because with his powers, there are a million ways he could make money, both legally and illegally, that would work better then bank robbery. Its really hard to take a villain seriously, when that villain seems so uncreative with their talents, that they don't even live up to a fraction of their talents. There was a recent Daredevil story, where a villain named Coyote kidnapped Spot and managed to steal his powers with the help of a mysterious benefactor and he used those powers in far more creative ways. He made millions in the drug trade, by using his powers to transport drugs across the world and got involved in the human smuggling trade. Even Coyote is not a good Big Bad, considering he is just a thug for hire, but a clever one. We have to stop treating bank robbery as the height of villainy, there are so many more interesting a villain can do than that.
Of all the villains who have not appeared yet, Mysterio and Kraven are the most obvious Big Bads who could carry one movie, not several, so trying to stretch out Kraven's story over the course of several films till we finally get to the good stuff seems like a bad idea and I think Mysterio needs some reworking because is not a very compelling character in the comics, usually. Vulture might work as Big bad, but they have to rework him a lot, up his power and make him more actively intelligent, don't have him engage in the same smash and grab tactics the dumber villains engage, give him some real clever schemes and pair with a physical villain who couldn't carry a film, like Scorpion.