The Hill notes that "The 2020 presidential election could feature the most crowded Democratic primary in decades, with scores of Democrats rumored as potential contenders." They've come up with a list of 43 possible candidates, including many from outside the political sphere, as Trump's success essentially opens the playing field to just about anyone with name recognition and money.

Here are the 43 grouped into categories, some within a section may no longer be in that role. Some names are much more realistic than others, but at this early date the article has cast a very wide net.  

• Vice-Presidents (2):  Biden, Gore
• First Ladies (1): Obama
• Senators (14): Booker (NJ), Brown (OH), Duckworth (IL), Feingold (WI), Gillibrand (NY), Franken (MN), Harris (CA), Kaine (VA), Klobuchar (MN), Murphy (CT), Sanders (VT), Warner (VA), Warren (MA), Webb (VA)
• Representatives (4): Ellison (MN), Gabbard (HI), Maloney (NY), Moulton (MA)
• Governors/Lt. Gov (10): Brown (CA), Cuomo (NY), Dayton (MN), Dean (VT), Hickenlooper (CO), Inslee (WA), McAuliffe (VA), Newsom (CA), O'Malley (MD), Patrick (MA)
• Mayors (3): de Blasio (NYC), Garcetti (Los Angeles), Landrieu (New Orleans)
• Other Politicians (3): Castro, Newsom, Perez
• Business/Entertainment (6):  Cuban, Johnson, Sandberg, Schultz, Steyer, Winfrey, Zuckerberg

The article does not delve into possible 2020 primary opponents for Trump. Whether any serious intra-party challengers emerge for the president will, of course, be largely dependent on his popularity and performance. This op-ed discusses some of the history around sitting presidents who have had to face a competitive primary.