Elizabeth Warren might be a candidate. Problem for the democrats is that they need to understand who their base is. Much of the traditional democratic base has shifted to the republicans.
This article by George Friedman helps to understand the shifts that have taken place in the republican and democratic partiesL
The framework of the Democratic Party was set in the New Deal. It was a coalition of Southern whites, Northeastern industrial workers, and African Americans. There were other elements, but this was the core. It was an unlikely coalition. In dominating the American South, it included white supremacists and African Americans in the same coalition, and then threw Catholic and Jewish Northeastern and Midwestern urbanites into the mix.
This is really going to be great fun watching the Dems try to find someone that can placate the looney-toon left of Crazy Bernie/Fauxcahontas, while accomodating the race/identity hustlers like Sharpton, while at the same time making an effort to woo back the droves of blue-collar voters who flocked to Trump.