Just let Biden VP again. Can Obama do it (semi-serious question)? The thing about nominating a progressive liberal is that there really aren't many high profile Democrats that are known to be progressive on par with Sanders or even Warren. Democrats have shied away from the kind of arguments Sanders has been making, at least for all my adult life. I don't know really anything about Castro except that he's young, Hispanic, and from Texas. Brown, I've seen on TV a few times, and he seemed more genuinely liberal than most, but who knows, and evidently, he had a falling out with Sanders, suggesting that if Sanders has any leverage it won't be Brown. Other names floated have been: Corey Booker, Devon Patrick, Martin OMalley, Tim Kaine, Mark Warner and Thomas Perez (current labor secretary, I literally know nothing about him). I've also seen both Minnesota Senators mentioned... I think her pick in some way must rebut the establishment attack, whether that's a non-politician or a liberal, I'm not sure, but if she merely reinforces what she already offers (say Kaine, Warner, etc.) I think she'll have made a mistake.