Ok, so this idea I had originally written in a comment on a video comparing the finales of all three shows, with some editing. Spoilers for all three, of course

So, at first the show is advertised as a sequel to TOH, with the trailer and promo material indicating it will be about a towards-the-end-of-university aged Luz and her friends exploring the Isles and helping build a new society, with the ultimate villains hinted as being the Archivists. However, by the time the first episode has finished, Luz and Amity have already been approached by the Collector to travel to the Archivists' main residence to spy on them (as he suspects they're up to something not good and they typically ignore mortals in their home). The fans begin suspecting something is up.

Then, towards the end of the second episode, Luz and Amity overhear a call between a senior Archivist and a mysterious person that the Archivist seems very deferential and respectful towards. At the very end, we hear this "According to the monitoring charms placed on the planet, the host has nearly fully charged the soul shard for harvesting. Soon your soul will be complete once more, Lord Cipher!". Cue a brief cut to Bill Cipher, laughing in a void, then cut again to a young man, waking up from a nightmare, forehead covered by sweaty hair. He seems familiar. He stumbles out of bed, grabs his phone from a desk laden with papers, books and notes and calls a number from memory. When the person on the other end answers, the young man says "Grunkle Ford? I had another vision. I think He's not as dead as we once thought." As he says this, he wipes his brow, revealing a birthmark in the shape of a certain constellation. Cut again, to a bedroom, with two women sleeping on the bed and a third one, with brown frizzy hair heading to the bathroom. We switch to her POV of washing her hands, when suddenly, a blue spark dances along her fingertips. The head jolts up towards the mirror and not only does her face look familiar, but her hair and eyes are starting to softly glow that same shade of blue. Finally, we hear coming from the bedroom "Um, Anna Banana, is that what I think it is? Because it isn't just you..." Head turns to see the two women from the bed now sitting up facing the one in the bathroom, undergoing a similar process, but with pink and green instead. Finally, we cut to the original title screen before it glitches, Gravity Falls style, into "Owlphibia Falls: Cipher's Revenge" and the real trailer begins. Cue the fandoms losing their shit.

Now, for the lore/Cipher's plan. When Cipher was killed, a part of his soul was sent through space and time by the Axolotl before melding with King while he was in his shell (this is the meta reason why him and Cipher sound similar). The Archivists receive a future vison from Bill with a deal: Make sure the piece is able to "recharge" and when it is ready, harvest it (which would kill King) and in return, Bill will give the access to many dimensions and realms to Archive. As for Amphibia's side of the deal, Bill can't harm the Axolotl for the soul splitting, but he can attack their protege: The Guardian. That, and he knows that the Calamity magic could cause him some strife if it was left in the hands of heroes on Earth. So, he made a copy of the Core's mind just before Calamity Anne destroyed it and made another deal: Make an upgraded version of the mind control devices, use them on the Trio and cede Earth's future control to him. In return, a portion of the Calamity powers will be given to them and Bill will direct them towards many other dimensions for the conquering and stripping of resources.

This show is set roughly 6 months after Amphibia's timeskip at the end, thus why Luz is nearly done with university. Yes, Sashannarcy is a thing.

I'm good at making cool settings/concepts like this, but bad at actually writing/drawing them. So please, if you're interested in expanding this, feel free to do so, just leave a link in the comments.