King Arthur was Arthur KirklandThe entire Arthurian Mythos sprouted from France getting his hands on England's diary and rewriting it to include more chivalry and more sex. "Mordred" might have been a dream England had of the future American Revolution, but got worked into the story for drama.
One or more of the characters is a Time Lord.
Guinevere is sterile.None of the Arthurian myths feature Arthur and Guinevere having children. This is strange for a royal family, especially one without access to contraception. We know Arthur is fertile; obviously, Guinevere isn't. This explains why Arthur is so passive about Guinevere and Lancelot's affair. He knows there's no possibility of Guinevere bearing Lancelot's child and affecting the royal inheritance.
Guinevere isn't sterile; it's just that King Arthur is female.As you know, Saber was once King Arthur. Two girls couldn't have a baby even if they tried.
King Arthur is Mordred's birth motherHey, who would dare call "him" on the pregnancy? Guinevere is busy, and Arthur is the King! We don't know who the father is. Given what kind of person Mordred turns out being, we can't rule out incubi. Or maybe there is no father — the person named as Mordred's mom in the legends tends to be either Morgan le Fay or Morgause, and both of them practiced magic. Of course, this means that either magic has to create a Y chromosome or the gender-flip is also hereditary.
In addition of King Arthur being female, Guinevere is male too, and he crossdresses as a princess, and he is a convincing Wholesome Crossdresser.All this is a conspiracy. Well, in this case, they can have children, but King Arthur will be the one who can be pregnant.
And with the WMG above, Lancelot is a gay.Boy x Boy love between Guinevere and Lancelot can be interesting.
King Arthur already returned in England's time of greatest need.World War II seems to be the most likely time, and Churchill the most likely candidate for Arthurship.
Morganna le Fay loved ArthurThink about it: she's supposedly Arthur's greatest enemy, hating him because he is the product of Uther Pendragon's rape of their mother. But what does she do to him that's so awful? Well, she exposes that his wife is having an affair with someone who is supposedly his friend. Then she herself sleeps with him, finally giving him the son and heir he had longed for for years. Then, when he lies dying after killing their son, what does she do? She takes him to Avalon to heal and someday return.
King Arthur will come backBritain's hour of greatest need will come during a third world war fought between a revamped British Empire and the United States in the 2050s. Arthur will return and lead the British in an invasion of America. However, this will also count as America's hour of greatest need and Captain America will return to lead the United States. The unstoppable Excalibur will strike Cap's unbreakable shield, and existence will explode.
Mordred managed to have kids before that last battleIt was decided not to let one of them take succession when that last battle happened. But the line survived in secret until the legend faded into, well, legend. Every MacArthur, McCarthy, and McCartney out there is a testament to this, and many of them even carry a little of King Arthur's blood... as well as a little of Mordred's. (It can't be helped.)
All of Arthurian Legend exists in a MultiverseThis explains the various interpretations and conflicting facts, as well as the retellings. All of the retellings of the mythos, from The Mists Of Avalon through the Merlin television series are true, just in alternate universes.
Excalibur is a reforged CaliburnSee, the Lady of the Lake named it EXCaliburn, which became Excalibur due to language drift. EX meaning Extra, as in extra powerful, of course. I guess saying it was formerly Caliburn works too...
Merlin is from the futureThis explains why he knew what was going on in the future. For the dragons, he just used seismic machines to find out they were there.
Merlin is John the Beloved.Okay, think about it for a minute. Jesus tells Peter not to worry about it if John stays on earth until He comes back, and tradition holds that that means that John's immortal. John had a vision of the future in Revelations, and was one of Christ's twelve Apostles, with Christ's authority and power. Why would a pagan wizard be an advisor to a Catholic king? And how would an immortal miracle-worker with knowledge of the rest of history who didn't want to be found out explain himself? "I'm a Wizard! These miracles are magic! Oh, and hey, Arthur, we should make England a better place, huh?" Oh, and he told Arthur that the two of them would be coming back to England one day, after their "deaths". Which sounds a lot like the resurrection an Apostle would be preaching. John the Revelator was totally Merlin the Wizard.
Arthur and Guinevere never had sex.This theory was actually put forth in J. Robert King's (sadly almost unknown) books Mad Merlin, Lancelot Du Lethe, and Le Morte D'Avalon. Basically, Guinevere was one of the Fae, and her magic gave power to Camelot. Arthur loved her and wanted to marry her, but sex with a human would have destroyed her power. HOWEVER, Lancelot was actually a changeling (the human son of his father had been switched out), and thus a Fae as well, so she could have all the sex she wanted with him. This version of the story also allowed for reconciliation of Lancelot's nobility with his adultery: he didn't commit adultery. The Fae allow polygamy, and he was Guinevere's second husband...only Arthur didn't accept this as legitimate, which led to a war between Camelot and Lancelot's kingdom of Brittony which paralleled the story of Helen of Troy.
Relating to the "Arthur was a woman" theory, Guinevere never actually existed, and Lancelot was in love with Female!Arthur.Unfortunately for Lancelot, Arturia did not return his feelings. As for Guinevere, she is a purely fictional character added to the Arthurian legend several centuries after the Battle of Camlann. People remembered that Lancelot had loved a beautiful woman, but since almost no one knew that "Arthur" was a girl, the role of Lancelot's Love Interest went to the King's supposed wife.