I think they should make a heroines line-up. Add girls like Kida, Jane Porter, Meg, Esmeralda, ect. Maybe not market it exstensively but try it out. I think it would be good for the fans, merchandise, and the movies. With an increase in character popularity it could/would increase the movie popularity. All kinds of possibilies could open up.
I wouldn't add anyone to the Dinsey Princess line-up that already exsists though, except maybe Merida or future princesses/considered princesses from films to come. The group now compliments each other very well and is cohesive.