Cory Booker is too much of an establishment politician, and as the second African-American nomination in a row, the Democratic Party might frighten away many of the mostly white voters. Bernie Sanders is good, although his policies are quite extreme and scare many moderates away, not to mention that not many people want a president who will have passed the average life expectancy.
I believe Elizabeth Warren should be the Democratic nominee 2020 because she has true talent and can bring together both the establishment and the anti-establishment Democrats. She grey up in a working class family in a small town in Oklahoma. Her mother kept the family afloat with a minimum wage job, even with her dad having expensive treatment. Warren's campaign will focus on the renewal of the middle class and the end of corporate/bank greed. Plus you can rely on her to be right because she was a professor at Harvard. The only arguments against her are "Pocahontas" and "you're rich, how can we trust you". She is a millionaire because of her years at Harvard the many books she has published, but atleast she isn't a billionaire like the current president.