Petra?
Personally, Phyllis Wheatley, Benjamin Banneker, MLK, WEB Dubois , Ralph Bunche, or Frederick Douglass could have also good candidates.
The new addition was specifically meant to be a woman, to celebrate the bicentennial of suffrage for women.
Harriet Tubman being on the $20 will not have any effect on my life, but she is a worthy choice. All historical and political figures have debatable
angles in their myths. Whether or not there are white men who achieved more than she did is precisely the point. Especially during her lifetime, this country was designed to keep people who weren't white men from aspiring to and achieving any sort of greatness.