He'd be best off going in as the VP candidate in 2020, to increase his national exposure, while the Democrats exploit and expand his contacts/ability to raise massive amounts of cash. He needs to stay away from getting into the mud with the current crop of insane bottom-feeders lining up for the Democrat nod. The extreme leftist bullschiff they're aiming for isn't going to last long.
Being a VP candidate keeps him insulated from having to make a record of policy preferences (which can be used against him later), and a loss to Trump doesn't hurt him (as it was the nominee who lost to Trump, not Beto.) That puts him in a prime position to run in 2024, without an incumbent (and severe GOP White House fatigue), if they lose, and puts him in line in 2028 if the Democrats actually manage to win.