Assuming 2020 is shaping to be a good Dem year and/or Trump decides he wants to be a one-termer (However, I don't think those occurrences would be mutually exclusive, given Trump is conceding to #TheResistance that his term was disastrous):
Nikki Haley
Mike Pence (assuming he doesn't get implicated in any of the Trump investigations)
Charlie Baker
Tim Scott
Justin Amash
Tom Cotton
Brian Sandoval
Susana Martinez
Assuming 2020 is shaping to be a good GOP year (HIGHLY unlikely):
John Bel Edwards (this guy should definitely wait...he would have to kick off a 2020 campaign before the 2019 gubernatorial election)
Ralph Northam
Kamala Harris
Kristen Gillibrand
Joe Kennedy III
Chris Murphy
Steve Bullock
libertpaulian has pretty much got it I think.
Northam I don't think would really run. Other than that I'd add Eric Greitens to the GOP list and Seth Moulton to the Dem list
