This has probably been done before by someone with more swag than me but SVSSS AU idea:
Shen Yuan is reborn as one of Binghe’s many forgotten wives in his harem, he makes it his mission to divorce this demon emperor no matter what, turns out PIDW has extremely elaborate (and weirdly horny) super fleshed out divorce laws (because a lot of the members of Binghe’s harem were already married or betrothed) that make it impossible for Shen Yuan to divorce the demon emperor, this will not stop him from trying.
Shen Yuan becomes the top expert in divorce law for demons, mortals and cultivators alike! He personally may not be able (yet) to get his divorce (this is airplane’s fault) but he is giving consultations left and right, divorce has skyrocketed and it is all Shen Yuan’s doing!
Binghe is just fascinated by this wife who’s name he doesn’t even remember just getting really in to wrecking other people’s marriages one day all of a sudden, he hasn’t laughed this hard ever really, this silly wife of his is truly a treasure and can make him smile genuinely like he had lost the ability to do.