(1) Sybilla Corbet. Mistress of Henry I (one of the many), mother of his illegitimate children (again, one of the many). She married Herbert Fitz Herbert, whose father was Henry I's chamberlain, and apparently bore children to both her husband and King Henry over a period of thirty years. She must have had considerable influence with the king, since her eldest daughter, Sybilla, became queen consort to Alexander I of Scotland.
(2) Alice Perrers, mistress of Edward III, who supposedly stole rings from Edward's body while he lay on his deathbed. (Though this is likely a false rumor.) Fortunately, I just read that Candace Robb is at work on a novel about her. According to her online newsletter, A Gift of Scarlet will be published in the UK in April 2009. That's three long years away. I will be patient.
(3) Isabelle of Angoulême, queen consort to King John. Jean Plaidy has written a novel about Isabelle's rivalry with the French queen at the time, Blanche of Castile (The Battle of the Queens), but surely Isabelle deserves her own novel... one that covers her marriage to the son of her former fiancé, Hugh of Lusignan. Besides, in every novel I've read about her, she's portrayed as a heartless bitch.
(4) Joan of Navarre, queen consort of Henry IV, widow of the Duke of Brittany, who was falsely accused of witchcraft after Henry's death. What more potential for drama could any writer need? Witch Queen by Maureen Peters is a novel about her life, though it's short and rather superficial.
(5) Mary, the daughter of King Stephen who became Abbess of Romsey. Abducted against her will by Matthew, Count of Flanders and Boulogne, Mary was forced into marriage with him. They had two daughters before she returned to her much calmer life at Romsey. Her story certainly offers plenty of drama, though the romantic aspect might be lacking. But if you can't wait for a more in-depth novel about her, why not read Princess, Nun, and Wife by Judy Walker, which Romsey Abbey sells in their gift shop for £6.99.
Who else would you like to add to the list, royal or not, medieval or not?