President Andrew Jackson should be removed from his place on the $20 bill. The Indian Removal Act only narrowly passed Congress, but when an 1832 Supreme Court ruling found it unconstitutional, Jackson ignored the ruling and continued with what would become known as the Trail of Tears, thereby leading to the deaths of thousands of Native Americans.
All the other faces on America's currency were indispensable to its founding. The Rev. Martin Luther King, Jr. was likewise indispensable in our modern era.
King is a universally beloved icon. He was a community and religious leader, an anti-war protester, and a champion of civil rights. He wasn't a president or a member of the ruling elite like Hamilton and Franklin. He was a normal citizen that changed America. A modern Founding Father.