I just read an article that threw out the idea that the title Prince and Princess could be used for those who are needed or chose to be a working royal. So basically, the monarch could "appoint" a member of the family to be a prince. So, it might be that we have a Princess Charlotte who is a working royal or a Louis Mountbatten-Windsor who is not. Maybe a Princess Louise who was formerly a Lady. What an interesting idea. I'm not sure how I feel about it.