As I've mentioned before, when The Collapse happened, culturally, people tended to retreat into different spheres--the Middle American culture that favored faith-based content, westerns, and odes to the U.S. Military. Then there was the High Culture that is favored by whatever passed for the aristocracy--stuff like Masterpiece Theater, Opera, and the occasional modern artistic endeavors. Stuff that thrived in late stage capitalism, particularly "geek culture" definitely took a big hit.
That said, a corporate and cultural behemoth like Marvel didn't completely disappear. Spider-Man is not unfamiliar to most people, and Marvel's depiction of Thor and other characters from Norse mythology have ended up creating a sort of symbiosis, with Thor sticking around as both a piece of historical and modern popular culture.
Marvel comics have also created a group of characters that, while not public domain, have namesakes and connections that have been absorbed into Marvel's mythology. Basically, I looked for iconic Marvel characters who are usually not the first that come up in the google image searches, have connections in fantasy fiction.
White Queen/Hellfire Club: besides the White Queen featured in "Alice in Wonderland" (who in turn featured in the recent Disney movies), there's the novels and television mini-series about Elizabeth Woodville--curiously written by Emma Frost, the name of the Marvel character.
The Hellfire Club is a real organization, once including Benjamin Franklin in their ranks, and has been the subject of many conspiracy theories.
Black Knight: The Black Knight is a mantle held by numerous Marvel characters, the most prominent being a member of the Avengers during some of the team's largest battles. "Black Knight" as a concept has featured numerously throughout fiction, most notably "Ivanhoe". It's also quite possible in some regions, the character/archetype has been conflated with Batman.
Beast: A longstanding member of the X-Men, who's appeared frequently throughout its media (funny enough as a usual latecomer that becomes a staple due to the character's "mascotability" and scientific acumen). "Beast" would also be known from the "Beauty and the Beast" story, which Disney has depicted twice to great success.
Fly: Occasionally Spider-Man has tangled with this character. But Most people know "The Fly" as the 1958 sci-fi chiller, as well as its 1986 remake. The Fly is a more explicitly science-fiction monster, but it's still included in many collections of monsters. Spider-Man doesn't hurt for archenemies that would fit in a medieval setting (To wit: The Green Goblin), but the natural animosity of predator and prey makes for good stories.
Destroyer: A very generic name--several Marvel characters have had that name, as well as a series of 1970's paperback novels. "Destroyer" is also sometimes used to reference the villain in "Ghostbusters". There's also the name of battleships. Also this combined funnels into the "character" from Thor's comics and movies--an animate suit of armor.
Enchantress: Another Thor adversary with a somewhat generic name, Marvel's Enchantress has been recently overshadowed by the one from "Suicide Squad" due to the movie--there's also a possibly less malevolent Enchantress from "Beauty and the Beast". Copyright laws generally lapsing over the centuries, however, stories conflated all three figures.
Firelord: One of Marvel's more obscure characters, though trading blows with the likes of Spider-Man and Thor, the concept of a "Fire Lord" exists throughout various iterations of fantasy fiction and gaming. Because "Fire character" is pretty common throughout comics, this catch-all name made him the most prominent.
Ghost Rider: A Marvel character with a pair of movies, and the subject of an old-timey Western song, "Ghost Rider" has become a generic enough concept that various sports teams have used the name. In fact, there's a Hockey team that evokes the Marvel image as much as possible without infringing on the trademark.
Hawkeye: The name of the Avengers character (and sometimes butt of jokes--though an Archer would certainly have a warm place in Neo-medieval stories), "Hawkeye" is also the smart-alecky lead of M*A*S*H*, and the nickname of "Last of the Mohican"'s Natty Bumpo. To say nothing of Iowa being named "The Hawkeye" state. It's easy to see how all these elements conflated into one character--a character who fought alongside Thor, among others.