Don’t forget the inclusion of The Marvelous Misadventures of Flapjack when Prismo is skipping through universe “channels!”

There’s also a blue jay that looks suspiciously and sounds suspiciously like Mordecai from Regular Show in the Adventure Time episode “Ketchup.”

Gravity Falls had a crossover reference with Rick & Morty. (GF would be the only non-Cartoon Network show in this theory)

Over The Garden Wall is canonically in the Clarence universe.

Samurai Jack has an episode that takes place in post apocalyptic Townsville from Powerpuff Girls.

We are getting closer to the grand unified theory of Cartoon Network.