I don't think we should be encouraging people who have served 18 months in the Senate to gear up for a presidential campaign. I think we should encourage them to gather records of service on committees, policy competency, and legislative records. Obama was absolutely underqualified for the presidency and that has given others the false impression that qualification doesn't matter just because he turned out right.
For actual names I think are poised to do something in 2020, here are some I'd float:
Gov: Hickenlooper, Bullock, Brown, and Malloy
Sen: Bennet, Klobuchar, *sigh* Booker, Gillibrand, Brown, Kaine, Cantwell, Warner, arguably Baldwin, and I'd have said Kaine but I sense the party wants to distance themselves from 2016.
Of the Obama cabinet, I think Duncan, Lynch, Rice, and Sollis could get there some day but won't be ready for 2020. I can't think of anyone in the house. There are a handful of ex-govs and ex-sens I think are well poised. Deliberately picked all people well under the age of retirement too, for bonus hip with the kids appeal.