Why don't they just put Esmeralda, Meg, and Jane in the line? Mulan's not a princess, but she's in the line. It'd be a nice way to not only allow young girls to know these awesome leading ladies, but teach them that you don't have to be a princess to be special.
I suppose it would make more sense for them all to be princesses since they all have admirable traits in their personalities. I personally love that Mulan is there even if she's not technically a princess because of her bravery. That may just be me! I love female characters that show that women can be fighters too. I do wish some of the other girls were in the lineup too, like Esmeralda who fights for justice and such. I think characters like her do show that it's not all about royalty, I just hope young girls can see that too!