I think they do just fine on dollar coins.
Why does any woman deserve to be the $20?
Maybe they could do a new $20 that has both Jackson and his wife on it. Of course she was never First Lady--she died between when Jackson won the election and when he was inaugurated.
Another idea would be to replace the men on the different denominations with a woman associated with them--Sally Fairfax on the $1 bill, Sally Hemings on the $2 bill, Ann Rutledge on the $5 bill (if there is any likeness of her preserved), one of Hamilton's married lovers (I think one of them was a Mrs. Reynolds) on the $10, Lucy Mercer on the dime, etc.