This came to me while I was listening to the song of the same name. It is set in the future when, as in the legend, Arthur is married to Gwen. Implied Gwen/Lancelot and Arthur/Merlin.
After Merlin came, everyone was happier. And there was one person's new-found happiness that Morgana noticed most of all - now why couldn't she make someone that happy herself? Morgana/Gwen, Shoujo-ai/Slash