It depends on the public perception of the success or failure of the Trump years. If, against substantial odds, the Trump economy somehow works for everyone, the Democrats will have to lay out a progressive vision based on youth, class, and race and involving significant institutional change; AOC immediately comes to mind here. If, on the other hand, Trump is viewed as a complete failure, the Democrats will play it safe, emulate the Labour Party in the last election, and pick the most moderate white guy they can find–a younger version of Joe Biden. Josh Shapiro would be a logical choice.