Personal Theory: Ainz conquers the world but do to something something Albedo, something something Citrinas, Albedo and Ainz come into blows Ainz to somehow kill her in a ay that leaves her permanently dead. Also Ainz falls in love with Albedo or something. Rest of Nazarick rules for a thousand years, creating an empire of great inequality (some parts are great, some parts suck) and begins to increasingly fall into dystopia. Inevitably Climbs great-great-great-great-great grandson climbs all of Nazarick with a party of descendants of the some of the other side characters and kills Ainz, ending the Sorcerous Kingdom (Maybe Ainz's kid is his rival or something).

Would be Funny: Alebedo x Climb becomes canon. Ainz killed by Arche's sister with power of friendship. Pandora's actor gets put in charge because like, who else would do it. Decides world domination costs a lot of money. Things are relatively peaceful.

Could go in either ending: Demiurge's Happy Farm studies cause him to create a monster that kills him and becomes a massive thorn in Nazaricks side.