Gonna go way out on a limb here:
K. Harris & J. Kennedy III for the Dems in 2020.
Harris talks tough. I mean she has a Trump-esque style of "I don't take no bulls...", but with better grammar. I have a feeling, that if she decides to throw her name into the ring, her attidude will resonate with the Dem base.
Kennedy. IMHO, the current political ethos (for lack of a better word) could go for some invigorating young blood. I look at Trudeau in Canada and am kinda envious that they have a spry young leader - someone closer to my age. Someone who seems to be more "anti-stupid" than anything. (When people ask me my political party, I reply with, "Anti-stupid") At least that the impression I get from him.
Plus, JKIII might want, need, or the movers and shakers behind the scenes of the Democrats might want to "season" him a tad bit.