But because we know the end, or think we do, care must be taken when the story is told. Hew too closely to the truth that already exists in readers' minds, and your work is simply derivative. Novelty is appreciated only if done with great care, and knowledge of the interstices in the Story that are waiting to be explored. Anything too novel, and your readers will wonder if you know anything about your source material in the first place.
Or there is the option taken by Catherynne M. Valente in her exquisite novella, Under in the Mere: to acknowledge that the Arthurian Story has become so large that the characters in it are no longer characters, but have become archetypes, roles to be played that are dictated by the bounds of the story. This acknowledgment is open - each chapter opens with a Tarot card (illustrations are by James A. Owen and Jeremy Owen) that indicates what role the character fills. And by telling the story in this way, by looking at what it means to be the Lady in the Lake, the Lancelot, the Morgan, Valente manages to do something new with the Matter of Britain, to make the characters both fully inhabit the roles that the story dictates for them, and, at the same time, to make them more than simply those roles.
Under in the Mere also deals beautifully with the fact that when a story is told as often as Arthur's is, that the later versions of the story overwrite the earlier ones. In other words, post-Eliot, a reading of the Fisher King's story is as influenced by "The Waste Land" as Eliot's poetry is by the story of the Fisher King. And so there is the imagery of the desert in Valente's writing, and her characters are haunted by modern day Camelots, and the knowledge that their stories have been told over and again. They know how this ends as well as we do. And so what they tell us is not the beginning and ending of Arthur or Camelot, but the way their story begins and ends. Valente's writing tells the truth of a Galahad or Mordred as a literary symbol who is also still a person.
I love stories about Arthur and Camelot. My bookshelves are full of them. Under in the Mere makes that well-known story new again, makes me remember why it began to haunt me in the first place, so long ago that I cannot remember I time that I did not know it. It's an amazing, gorgeous work. I expect that I will see it on a lot of "Best of" lists, and I highly, highly recommend it.