Well, actually, Elfwine…
Merlin as village idiot: Maybe not ‘Merlin’s official job is village idiot’ but as being the dumbest member in the cast, that’s been done, ESPECIALLY in parody versions like King Arthur’s Disasters. Mark Twain’s A Conneticut Yankee in King Arthur’s court plays that too in a serious way. And I’ve only seen a few episodes of BBC Merlin, but he was usually portrayed as being unreliable and kind of dumb, even if he was only faking it.
Sir Kay is Arthur’s Court Jester: Generally Dagonet is court jester, but if we’re going for ‘who is the useless comic relief’ type role in the cast, Sir Kay ends up as joke of the court quite a bit- him being the cowardly complainer people laugh at, or the bad at his job knight who gets defeated constantly to everyone’s amusement. He’s not even always seneschal- he was in charge of the kitchens only in Arthur Rex, though it’s been a while since I read that.
Arthur’s is a schizophrenic despot: Hoo boy, this pops up a lot- making the famously virtuous king actually an evil sob is a relatively popular reversal, ESPECIALLY if the ‘Mordred as a white knight’ aspect is included.
Guenevere a guy in drag: Okay, never seen this one specifically, but I’ve seen lady Guinevere pretend to be a man in at least two ‘Guinevere is a warrior/tomboy and wants to fight’ versions, and if there is an Arthurian story where she’s a man who dresses in drag I’m sure it does or will exist. Fate/Stay Night had a female arthur dress in male drag and pretend to be male. Going outside this realm, many Arthurians stories do gender switches, especially in fanart. Going into ‘the character is transgender’ there are a few Arthurian stories with the mains being transgender, the most recent one popping into my head being Jean-Luc Istin’s Lancelot series. Tristan from Camelot 3000 is a very famous transgender character (though due to the magical nature of the situation, it’s complicated).
Mordred a white knight: Jesus H. Christ, so many. Mordred as being a perfectly good guy who has been maligned is REALLY popular and recognizably Arthurian. A lot of modern famous ones go this route, esp. Mary Stewart’s.
Setting resembling medieval Sicily: Actually, Arthurian Lit has a very lovely and popular history in medieval Italy, and there are a variety that changes the setting to medieval Italy- or the setting is in England, but everyone acts with the same social customs and language of Italy so it’d be accessible to readers, kind of like how Ace Attorney game implants it into California. I’ve seen Arthurian stuff take place in a bunch of settings- the far off future of Camelot 3000, multiple theatre interpretations of Camelot that have it in a renaissance, barbarian/viking era, tudor-esque, etc setting. There are many American setting versions of Arthurian lit. The webcomic Arthur, King of Time & Space had a multitude of AUS- M.A.S.H au, Buffy au, western au, space au, modern day america au, etc, etc. The album High Noon Over Camelot is a steamunk space western and it’s the coolest thing ever!
S-Sorry to basically take this time to rant about different versions of Arthurian lit, but yeah- what I’m trying to get at, I think that these all are still pretty recognizable! Heck, a lot of these combine those aspects or have even WILDER stuff in them. For any of those suggestions that don’t have a perfect example in something, you can bet it will be made at some point!
A lot of these, like Mary Stewart’s and Arthur Rex and Conneticut Yankee or the Camelot musical are definitely considered of the same tradition of TH White and Steinbeck- I think that Arthurian myth is so long, so varied, that you can pretty much do whatever and people would still know it, and whether it’s considered as legitimate as the older stories depends on how the audience takes it. I mean, Lancelot’s technically an OC who just got really popular…in 500 years, maybe any of those things you listed will be considered part of the mainstream tradition and recognizable.
(ESPECIALLY Mordred as a good guy, that’s so popular already and been around for such a long while that I’ll eat my foot if it doesn’t become the norm at some point.)