Now that I've thought of it, there's something awfully appealing about that, actually! I think the particular aspect of soulmate AUs that always bugged me (because, for some reason, the worldbuilding aspects you mentioned never did, even though it's a completely valid complaint) is the One True Love For Everybody thing, which is SUPER DEPRESSING when you really start to think through the implications of it. And so, I really like the idea that people are just guessing that "soulmate" is what the mark means, but it's actually a simplistic explanation for a phenomenon that is, in reality, much more complex and mysterious.