At the moment, the only ones who seem plausible national contenders are Baldwin and Polis.
I don't see Hillary going with a lesbian as a running mate.
Polis is a well-regarded swing state congressman, although there was that flap about his belief that colleges should expel anyone suspected of sexual abuse.
https://reason.com/blog/2015/09/10/rep-jared-polis-thinks-colleges-should-bThere are some pros and cons. It could bait the Republican ticket and surrogates into saying something stupid or offensive. But it could also piss off centrists if they think someone's sexuality helped them to the top of the ticket.
Mark Takano has some political talent, but he's too green as a second-term Congressman.