Castro is a little boy with no quality, he is never going to be the President.
This. And he is simultaneously boring and ugly.
As for me, I would pick Brown/Gillibrand. Sherrod Brown has this goofy grandad vibe that could be a breath of fresh air after Trump. Exemples:
https://twitter.com/ConnieSchultz/status/536553099318800385,
https://twitter.com/ConnieSchultz/status/227025045690646528.
Overall, his wife's twitter account is a goldmine of feel-good family bliss. He could really get that narrative going.
Personally, I love Sherrod Brown, but he isn't that charismatic and I'm not sure he's ambitious or even self-confident enough to mount a presidential run. In my opinion though, only Al Franken, Elizabeth Warren and himself could carry on the legacy of Sanders' 2016 historic campaign.
Kirsten Gillibrand could be a good running mate. She's young and her focus on women's rights would be a great symbol against Trump if he is seeking reelection.