Also, apparently Alex Hirsh confirmed during a livestream that the Bill Cipher appearances in The Simpsons are canon. Timeline-wise, this wouldn't work since Bill is meant to be trapped in the Theraprism, though I have two solutions for this.
1: Alternate universe (the boring option)
2: The season 2 finale was originally going to feature a song sung by Bill Cipher called "It's Gonna Get Weird", in which he sings "Look at that money. Who's that, honey? Look throughout history, how could you miss me? (Seriously, I am all over the place!)" The storyboard sequence for this song shows Bill pointing at the infamous triangle on the dollar bill in the first line, then other triangles with eyes throughout history. This may imply that Bill simply had a hand in the production of The Simpsons, or that Bill's influence on the world caused him to appear in the show (depending on how you interpret Bill appearing in history). Either way, due to the meta nature of numerous shows on this timeline, I'd have to include Gravity Falls on this timeline. Since I'm including Adventure Time, despite being an alternate universe, due to the multiverse nature of many of the episodes, it goes without saying that I'd have to include Rick & Morty too, due to the multiversal connection between that and Gravity Falls. That one sofa gag is finally canon(?)