Diana's hair will change a third time.
At the beginning of A Discovery of Witches, her hair is a medium blonde, straight, and shoulder length. Immediately at the start of Shadow of Night, her hair changes to be strawberry-blonde, curly, and waist-length. In The Book of Life, her hair will undergo a final change and become platinum blonde. This will be Lampshaded and possibly compared to an obscure alchemical process. Matthew's vampirism will be undone.
Peter Knox drops hints that Ashmole 782 tells of how vampires were first created, and how they can be destroyed once and for all. Besides, what kind of a happy ending is a Mayfly–December Romance? (Which raises the second question, who said this was going to have a happy ending?)