Eleanor Roosevelt rejected the idea that First Ladies should be prim and proper arm candy. She publicly advocated for Civil Rights, Women's Rights, and more, often completely independent of her husband. She probably would have made a strong presidential candidate, but since a run was out of the question for her back then, a $10 bill spot may be the honor she deserves.
Elizabeth Cady Stanton was one of feminism's OGs. She was one of the most vocal suffragists, abolitionists, and womens rights activists back in the 1800s -- when that sort of thing didn't exactly sit well. She probably never would have thought a woman would wind up on the $10 bill, but few are better suited for the honor.
Along with her friend Elizabeth Cady Stanton, Susan B. Anthony was one of the forerunners of the suffragist movement. Like Sacagewea, Susan B. Anthony graced a $1 coin, which was discontinued due to unpopularity. A bill would make a better home for her.
Aide from being the first deaf blind person to receive a bachelor's degree, Helen Keller was an outspoken activist for workers and women's rights. From being unable to communicate whatsoever, to gracing the $10 bill...that would be quite a symbol of the American dream.