There are many links the the Arthurian myths in the Dresden Files, and not merely the traditional myths but the many retellings and adaptations. I am just throwing thoughts out of my head to work them out here - and will clean this up soon.
There are several archetypes/roles in the Arthurian Legend:
The Rightful King - Arthur. In Dresdenverse, this would be a likely someone like Michael, though Arthur is probably more fallible. Interestingly, as the story became more Christianized the idea that Arthur became "Rightful" rather like God/Jesus being the true King. This is important. Marcone is also a candidate - though that would be quite dark.
The Wizard/Druid - Merlin. Clearly this is Harry, he is not meant to rule but to be a part of events. Now obviously there is/was an "original" Merlin who began the White Council and changed the world, and excluding Time Travel and working with the fact that he is dead, Harry Dresden is clearly being lined up for this role.
The Enemy - Morgan(a) Le Fay. Again, clearly already exists/ed in DV but assuming she is dead the current candidates are not so easy. A few candidates stand out: Elaine (assuming she is evil), Kemmler (bear with me - the evil Wizard behind everything), an unknown threat.
The Weapon - Mordred. His whole purpose is to be the weapon that kills the rightful king. Nothing like the power of patricide. Harry could fit this role, as he has often been described as a weapon and used as such. Also possibly Marcone, though he feels a bit old for the role. However, more likely this will be something out of left field - maybe like a future apprentice of Harry's/child (for a guy who doesn't get much, he sure seems to keep having kids!)
The Knights of the Round Table - Clearly the Knights of the Cross fulfill this role.
Nimue, the Lady of the Lake is a central and important figure. She is Lancelot's fairy godmother. She is notable for giving Arthur Excalibur and giving Lancelot the Holy Grail. She refused to give Merlin all her love until he taught her all his secrets, and then she locked him away (originally either beneath a stone or in a hawthorn tree - pay attention, in DV Dresden gets stung with a hawthorn dart during Cold Days and there are several significant stones (Stone Table, Stone Statues etc). In other versions it is an invisible tower or a cave (Demonreach - and yes I know Jim says it isn't Merlin in the crystal, doesn't mean he isn't down in there). Here is where it get interesting! The goddess Diana (Artemis) is Nimue (also known as Vivenne)'s godmother and Nimue's father is...the god Dionysus (Bacchus)! Interestingly, the Lake she is associated with is sometimes considered the Lake of Diana in Sicily - where Queen Diana (who was worshiped as a goddess) was murdered. This is where Lancelot du Lac (of the Lake) was raised. In some versions - Morgan Le Fay is Nimue too residing in a town named Ninniane (one of the names of the Lady of the Lake). In the Hellboy comics, Nimue was locked away and arises to supplant Hecate as Queen of the Witches. She is the Queen of Blood and takes up the mantle of the Morrigan and leads an army of legendary and folkloric beings. In the 80s novel "The Mists of Avalon" the Lady of the Lake and Merlin are offices in the pagan heirachy, Merlin is a young druid and Lady of the Lake is the ruling priestess of Avalon. Multiple characters hold the mantle of Lady of the Lake including Morgan Le Fay, Vivienne, Niniane and Nimue.
In Dresdenverse who is the Lady of the Lake? - Leanansidhe seems a bit on the nose, so it leaves a few options to who the original was. Mab (when she was Lady) is possible, she would have been quite young as that would have predated Hasting by several hundred years. It is possible that Morgan Le Fay literally held the Mantle of the Lady of the Lake (whatever that mantle actually is) and either gave it up or died - and so have several other beings. Perhaps the mantle is one of the six queens, perhaps something else. Perhaps Nemesis the Greek Goddess is "Nemesis" and Hecate is the six Queens of Faerie and everything is just them having a shit fight...Nemesis is "Outsider Nemesis" is Nimue, uniting the formor and intends to supplant Hecate, who are the Queens of Faerie.
Other interesting connections
Elaine Mallory - Interestingly close to the Summer Court, shares a name with Thomas Malory (who wrote Le Morte d'Arthur - which the myth of King Arthur is based), candidate for being Kumori. Could she be the Morgan Le Fay archetype? Interestingly, as she was Arthur's half sister and also the mother of his bastard son and killer Mordred, considering my theory that Harry and Elaine are half sisters her candidacy gets stronger. Also is the name of the DAUGHTER of the Fisher King. The WOUNDED KING'S name is Pellehan (named Pellam of Listeneise) in Malory's Arthur, the Fisher King is Pelles. Pelles tricks Lancelot into sleeping with his daughter Elaine, in order to sire Galahad (the perfect Knight). WTF!!
John Marcone - Not much known, pretty powerful and successful person - made more headway in someways than Harry ever has (all the more significant because he is vanilla). Could have been an excellent Monarch in ages past.
Ramirez - Feels like a Gawain (always a brotherly role with Arthur)
Butters - Probably Sir Gareth the Lovely Hands - knight of the kitchen.
Thomas - Lancelot. Hands down. Definitely mister steal your girl.
Who is the Fisher King - the heart of Western Civilisation? This is so significant I am not sure what even to say...In some versions this King lives in a castle with his son (but they are both the Fisher King - very Father and the Son) relying on the Grail for sustenance. He bleeds from a wound in his thigh (this is very Jesus). In some versions, the wound was caused by the Spear of Longinus. In others, the Spear is used by Galahad to heal him. In Joseph of Arimathea (the man who caught the blood in the Cup making it the Holy Grail) the "Rich Fisher" is called Bron (Bran is sometimes an original spelling). Bran the Blessed in Celtic Mythology has a cauldron that can resurrect the dead. Very interesting considering the Saviour returns to life. King Arthur actually retrieves that Cauldron in one story.
Also what is the weird connections between Christianity, Greek Mythology, Irish Mythology and Arthurian legend? Strongly exists in the real world, and in DV seems even more connected. Obviously all part of Western Culture the past few millennia but still. Greek Gods connected to fae, connected to Wizards and Druids, connected the Grails and Christ, connected to Tuatha de Danaan. It just goes round like a pretzel!
Wow this really got away from me. I am gonna tidy this up.