The reason I don't tend to read these fics is because there's no explanation or world-building like this!
I think the only "soulmates" type fic I stuck with was Bound and Determined despite the D/S world (which doesn't really interest me) because it was all about the world-building! The characters didn't know why things were the way they were, but there were lots of theories, people researching from social, historical, political and scientific perspectives, personal opinions, good and bad outcomes for individuals, and a society that was plausible under the circumstances. I really liked that there was no single truth about why things worked out this way, just lots of conflicting theories, as there are about our societies in real life. It has a Charles/Erik focus, but a cast of thousands which really helps extend our view of their society, too. Definitely worth a read if you like this kind of world-building.
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I think the only "soulmates" type fic I stuck with was Bound and Determined despite the D/S world (which doesn't really interest me) because it was all about the world-building! The characters didn't know why things were the way they were, but there were lots of theories, people researching from social, historical, political and scientific perspectives, personal opinions, good and bad outcomes for individuals, and a society that was plausible under the circumstances. I really liked that there was no single truth about why things worked out this way, just lots of conflicting theories, as there are about our societies in real life. It has a Charles/Erik focus, but a cast of thousands which really helps extend our view of their society, too. Definitely worth a read if you like this kind of world-building.