Ok so for the au
Would take place after the events of Steven Universe, that all pretty much being the same.
Also takes place some time after Gravity Falls.
After season 2 of Amphibia Anne and the Planters whilst trying to look for a way home find out about Gravity Falls and the weird stuff that happens there and go there to try and hopefully get back to Amphibia.
This is all happening around the start of The Owl house, and King is Bill Cipher reborn with a second chance at life.
Like how human things end up in the Boiling Isles due to leaks between realms, there was a huge leak in the Gravity Falls area to explain why all the weird things that happen there happen.
Not completely sure who would be the main villain of the story, but it’d have to be either Belos or King Andreas. Don’t know which would be better.