There's been a lot of stuff about whether the cartoons all take place in the same universe or not. I have 2 theories about this.
Theory 1[]
All cartoons exist in the same universe, though some may be at different times. This renders CN: PTE and PTE XL un-canon.
Adventure Time takes place in the distant future, one made possible by the events of Samurai Jack in which Jack prevents Aku from conquering the universe. However, this conflicts with The Jetsons, though may support the theory that The Jetsons and The Flintstones take place at the same time, the supposed reason for The Jetsons always taking place high in the sky and The Flintstones always on the ground. This also conflicts with the The Jetsons Meet The Flintstones movie.
Dexter's Laboratory, The Powerpuff Girls, and The Grim Adventures of Billy & Mandy take place very close to each other, i.e. in the sense that they are in neighboring cities, and Dexter's Lab possibly takes place in Townsville, with Edd, Ed n Eddy and Codename: Kids Next Door somewhere in Endsville, if this theory is followed.
Many of the Powerpuff Girls references in Foster's Home For Imaginary Friends make it hard to place it in the same universe, though some believe that the episode in which the Director comes to Townsville and makes a movie on the girls is what everybody in Foster's was watching. Additionally, in the PPG episode Imaginary Fiend, the girls create an imaginary friend to combat an evil one.
Theory 2[]
Only a few cartoons exist in the same universe, which I will list below with evidence.
- The Grim Adventures of Billy & Mandy, Evil Con Carne, Foster's Home For Imaginary Friends, Codename: Kids Next Door, The Flintstones, The Jetsons - See Grim & Evil; Mandy appears in the Foster's episode The Big Picture as a cameo; See The Grim Adventures of The Kids Next Door; Fred Flintstone makes a major appearance in an episode of Billy & Mandy which I don't currently have the name of; See The Jetsons Meet The Flintstones.
- Adventure Time, Samurai Jack, Powerpuff Girls, Dexter's Laboratory, Regular Show, The Amazing World of Gumball - After Jack went back to the past and defeated Aku, the future he went to was erased and the time flow went back to normal, leading to the adventures of Finn and Jake in the Land of Ooo in a distant future; The Powerpuff Girls are referenced several times in Samurai Jack, including but not limited to the theory that Jack visits a post-apocalyptic Townsville and the various talking dogs, as well as a ruined billboard with the Talking Dog from The Powerpuff Girls which was also seen in newer condition in that show; Both Dexter's Laboratory and The Powerpuff Girls feature many the same characters, i.e. Major Glory, Valhallen, and the Infraggable Krunk; Regular Show and Gumball both exist at the same time, but in different places.