Frankly, I don't see why anyone would prefer Castro the empty suit. Warren seems like the obvious 'good choice', while Franken and Becerra are the most underrated.
Castro is a clown honestly considering the VP position. The guy could never even be a Congressman, forget being a Senator & then having some accomplishments. The guy was a small time mayor when Obama picked him up & made him Secretary of Housing & Urban Development. The guy is atleast 10-15 years away from being a VP Pick. Jeff Sessions or Chris Christie would eat the boy's heart out & serve it to Clinton.
Tim Kaine is a top pick IMO & comes from a purple state in Virginia but way too conservative for my liking. But it is better for Hillary to run a conservative administration with Warren in the senate where she is of some use rather than be a lame duck puppet. Having Sanders-Warren in the senate & a conservative Administration will lead in the future to a Stronger Progressive movement.
As for Trump Kasich would have been best, Sessions would be good as Secretary of State. Kasich would have surely handed Ohio which is an absolutely must win. If trump wins Ohio & either one of Pennsylvania or Florida then he is within a shoe of the white house & may win a couple of small states to go through. No way he is winning without Ohio.