So let's realistically look at what a Universes Beyond could be.
MTG:UB comes in different sizes from a secret lair (a handful or cards, some re-themeed and some new) to a full set. I don't know what the D&D equivalent is but let's say it's the difference between a web supplement and a full hardcover book.
The second thing is that I assume the goal is to support D&D, not recreate the d20 heyday. So it's going to have to use the PHB classes mechanically unchanged. That also means the the setting of going to have to accept the classic D&D tropes: fighters, clerics, wizards and rogues. That concession automatically removes a lot of properties that don't (for example) use magic.
Third: it has to be a setting you can run a game in. At the very least, that means something that is self contained or lacks a direct conflict (like SpongeBob) probably isn't a strong candidate.
Lastly, WotC is very deliberate with new mechanics. Species, backgrounds and feats are a given, spells and subclasses are rare and limited, new classes are rare as rooster teeth. Absolutely no way they are creating a new spell system or other base mechanic. If you can't create a Jedi using the majority of the PHB, you probably aren't getting a Jedi class.
So what does that leave?
Magic the Gathering is a shoe in.
Final Fantasy would be an excellent candidate.
Lord of the Rings would probably work but 3pp seems to have that handled.
Ditto Game of Thrones.
Warcraft, the Elder Scrolls, and Dragon Age would probably be good choices.
As would the Witcher.
And maybe the Souls universe.
Harry Potter might have been a coup, but the brand of very polarizing now.
There are plenty of Anime and other video games that could work. I don't know how viable the licenses are, but Castlevania or Legend of Zelda could work.
Any setting that has modern technology or even futuristic would be viable as long as there is magic there too. New tech can be grafted on easy.
Superheroes would be tricky, depending on the type. Despite the popular belief D&D is a superhero game, it's really not good at recreating Spider-Man.
The biggest catch would of course be Disney. Kingdom Hearts type D&D setting?
Now a lot of these concessions are based on how WotC has handled it's listened products since 5e (MTG, Minecraft, Rick and Morty, Stranger Things). WotC could surprise us and buck tradition with Star Wars Saga 2 (based on 5e) but I feel that is unlikely.
Anyway that's my thoughts on this