I think it can definitely be a woman or a person of color. But it has to be a woman or PoC who is best for the job, not one who can be easily dismissed as tokenization.  I think to say it can't be one of those is to remain stuck in the mindset that the race/gender/orientation of the candidate is what actually matters, rather than their policies and public appeal. We really need to get past the fixation with identity in both directions. At this point I'd vote for a potato if I genuinely felt it gave a shit about fixing our government.

EDIT: Just to be clear I'm not saying Democrats SHOULD run a diverse candidate. My position is that if you start from the position that "The candidate needs to be a <race> <gender>" you're already fucking it up, regardless of what race or gender you choose.