King of Britain after Arthur
Constantine
The man handed the crown at the end, whom almost nobody remembers.
In both Geoffrey and Malory, Arthur gives the kingdom to Constantine, his kinsman, before he is carried away. It is a small detail with a large implication: the succession is completed. Whatever Avalon is, the realm has been formally handed on, and the government of Britain proceeds — which sits oddly beside the promise that Arthur will return to it.