From my transatlantic, disbelieving and totally ignorant point of view, I’d say the keys are: “representation” and “diversity in thought”, but maybe not in the sense you meant when you wrote them in the first place:

“Representation”, in the sense that the ideal candidate must be someone with whom the masses can or want to identify themselves with, so whereas Elizabeth Warren looks well on paper, she’s far to “academic” and would only appeal to the medium-high social stratum.

You need a more populist candidate.

“Diversity of thought” meaning “openness” and “malleability”, since as Darwin said: It is not the most intellectual of the species that survives; it is not the strongest that survives; but the species that survives is the one that is able to adapt to and to adjust best to the changing environment in which it finds itself…

This doesn’t mean that the candidate shouldn’t have a backbone, on the contrary it means that he/she will need to adapt to the environment, but keeping their identity and ideals, so they don’t delude their electorate.

In short, you need an artist for the histrionism, and for their freedom of thought. Whatever they do, it will look authentic and convincing…and people will be able to relate to them.

I’m not the first to say it, and you already have a bunch of actors turned politicians… maybe the Democratic Party can look for a Rock star :)