Remember though that Diana wasn’t allowed to be THE princess of Wales after their divorce. She was Diana, Princess of Wales. So as she wasn’t a queen herself, wouldn’t have needed any additional title.
Henry Tudor’s mother Margaret Beaufort titled herself My Lady the Kings Mother which they might have recycled to give her a formal connection to King William.
Interestingly, Diana also lost her HRH status when they divorced, about which she was reportedly very upset and it was said that william promised her that he’d give it back to her when he is king.
I wonder if he will do that posthumously when he is king? (Assuming is true of course!)