I've been having thoughts about Soulmate AU worldbuilding for a long time, both the mechanics of the magic system and how that magic system came to exist (hence that post), but my thoughts only organized themselves recently because I was scrolling along (I was not looking for soulmate fics) and came across a Soulmate AU fic for the DCU. It was some Batman fic, I think.
I've been rewatching the "Justice League" animated series and the "Batman: The Animated Series" cartoon recently, and in the former, magic is very explicitly a part of the DCU. There are gods and demons and wizards and sorcerers and other people who know magic all over this universe. This is where my mind went immediately upon being reminded of Soulmate AUs.
I think it would be funny if Dr. Fate knew exactly how soulmates worked and exactly who is responsible for this magic system. Like, "Oh, yes, soulmates were the work of the Inverse Dimension Immortal Being Asdfghjkl in the year 10,000 BC, and no one's managed to undo the spell yet, though the world has frequently nearly been cast into several apocalypses by people trying. People are not open to hearing this particular explanation, however."
Someone asks Zatanna to get rid of their soulmate mark and she's like, "Uhhhhh, sorry, but I like not having my head exploded by matchmaking demon gods, thanks. It's a personal preference."
(Is just this an Earth thing? Do aliens wake up with writing on their arm one day and go, "What the fuck is this?! Am I sick?!" And eventually they meet a (non-human) Green Lantern or someone who sees it, and that Green Lantern is like, "My condolences, but there's a human out there who has decided you're its mate." And the alien is incredibly pissed off about this, which eventually leads to a screaming fight upon the first meeting, where the unwitting human in question is like, "I didn't choose shit about this, you jackass! Stop destroying my fucking house!!!")
And if you want to do a world where soulmate marks are a brand new problem that everyone is being forced to deal with due to a worldwide spell, then you can just explain why this is all happening by saying, "It's John Constantine's fault." And I'll be like, "Ohhhhh, that makes sense. Please continue."