Who would you rather see first: Kraven or Mysterio?
Let's think that the movie execs understand they shouldn't stretch multiple villains into the picture, and should only focus on one villain. We know the next villain will be one we have yet to see on the big screen yet... The obvious choice is Scorpion, seeing as he is already cast, and already has a vengeance for Spidey.
There are plenty of contenders for Spider-Man villains who have yet to appear to have their moment to shine, but the most obvious has to be the final two members of the Sinister Six, Kraven and Mysterio... So, if you had to see one of them first, which one would you prefer to see?
They could always do something similar to Shocker in Homecoming, cast a lesser known villain as a Supporting Role... I could see Chameleon working alongside Kraven in a movie... And while I thought it was a funny gag and perfect casting for the unreleased Raimi movie, let's not make Mysterio into a gag villain by being unmasked as Bruce Campbell in the first act. (Though, I admit, I would have loved to see the Raimi films continue, only to call the full Sinister Six together, and Mysterio be the bumbling idiot comic relief villain.)