Regarding Dems in 2020 (waaayyy to early, but what else are we going to do before the Patriots season starts?) my preference for Lizzy Warren remains, but I have to say I was expecting her to emerge as more of a spokesperson for the Dem opposition. She really hasn't done that even from the vantage point of living in her state. Now perhaps she's taking it slow to focus on her re-election which is fine but I thought we'd get more of her making the rounds of the talk shows and calling press conferences to argue the Dem points. Right now oddly enough the most effective Dem politicians are both from NY

Schumer to my surprise has done a solid job running the Senate thus far and if he extracts some concessions in the debt ceiling negotiations (like funding ACA payments) then he's batting a thousand.
For governor's I see what Cuomo is up to, which is checking which way the winds are blowing and burnishing his liberal credentials. Whatever his motivations, he's passed same sex marriage through the legislature, banned fracking, and enacted a free state college tuition proposal. Check. Check. Check on a liberal's wish list and its an actual accomplishment.
IMHO the best job by any Dem governor today is Jerry Brown who has not only made California governable again but successfully run based on liberal principles. The dude's like 90, but he got the job done by any measure. However I believe he's done next year (term limits).