Fearless prediction: they’ll run Julian Castro, expecting him to lose, but using the election as practice for Cory Booker in 2024. (Or they’ll switch the two and run Booker to lose in 2020, and Castro to be expected to win in 2024.)
I'm thinking that Cory Booker will get a strong push and will be the nominee in 2020. The Democrats desperately want to find a "new" Barack Obama to recapture the White House, and feel Booker is the closest candidate they can find.