Why not Martin Luther King?
Probably considered too modern. But if you were to choose an internationally known representave of the US civil-rights movement, he would be my number 1 choice. He's the only one I really know of at any rate.
But, he's also a man. And I think some contention is a lack of female representation on the currency too.
While black americans make up around 15% of the population, Women make up 50%.
Harriet Tubman does seem at least a little bit like an attempt to hit two birds with one stone.