Actually, it's still undecided, but apparently it has to be a woman to "reflect the times" in which we live:
"A poll conducted by "Women on 20s," a popular campaign to get a woman on the $20 bill, showed that the leading contenders were Harriet Tubman, Eleanor Roosevelt and Rosa Parks."
http://www.cnn.com/2015/07/16/politics/woman-10-bill-rosie-rios/
Alexander Hamilton, should be the last to be removed from currency, given history.
I believe the picked the 10 dollar bill to deface, which would leave room for those to cry fowl at the "slave owning" Andrew jackson on the 20, so that it would be that much easier to convince people to remove him too.