The anime universe is vast and expansive, with every series portraying a distinctive world. Despite each reality's individual complexities, some of the worlds have a lot in common with other anime—so much so that it feels like some anime could exist in the same universe.
In a few cases, this is because of real world connections; the creators of Yu Yu Hakusho and Sailor Moon are married, and they definitely reference each other's creations in their works. Other times, the parallels have more to do with thematic similarities, like how Chobits and Plastic Memories are mutually fixated on robots with feelings.
Crossovers can illuminate motifs in both series, allowing fans to view each in a different light.
YouTube user Overmental believes all the Studio Ghibli films exist on the same timeline.
In Howl's Moving Castle, the first movie in this proposed chronology, a magical war wages with flying wizards used as weapons. These wizards look an awful lot like the flying robots in Castle In The Sky. While that film's robots don't appear in Nausicaä of the Valley of the Wind, other forms of technology, and the unique fox squirrel, do show up.
The impact of these wars may have led to a separation between magical beings and humans, which explains why humanity seems to lack a deep connection to the magical world in films like My Neighbor Totoro and Spirited Away.
This timeline spans hundreds of years, which is why films like Kiki's Delivery Service and Ponyo feature humans who openly interact with magic. Considering how all of these movies came from the same person's brilliant mind, it wouldn't be all that surprising if Overmental's theory ends up being correct.
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In My Hero Academia, heroes are rated (and compensated) based on a combination of tangible accomplishments and popularity. The pro hero system in One-Punch Man is pretty similar, but the requirements to become a hero vary.
While Saitama has to take a few tests and undergo some training courses, the process is less like obtaining a professional degree and more like getting a driver's license. Izuku and his friends, on the other hand, must go through a rigorous training program that requires multiple tests, internships, and practicum hours.
My Hero Academia takes place much further in the future than One-Punch Man, so it's entirely possible the MHA hero system is an improvement on the previously existing structure from OPM.
The Sibyl system is a government-run program that decides in advance who is likely to commit a crime and kills anyone deemed a potential threat. This is basically Light Yagami's fantasy come true. Sure, he can kill people with his Death Note, but his ultimate goal is a society where crime is completely eliminated.
Who created the Sibyl system? Psycho-Pass ever elaborates on this question, but it clearly began sometime between 2090 and 2100. Obviously, it wasn't Light Yagami, as he dies at the end of Death Note, and even if he had survived, he'd be 104 years old in 2090.
However, Light's legacy likely lived on after his death. Could his attempt to cleanse the world of crime have been the inspiration behind the Sibyl system?
Yoshihiro Togashi, creator of Yu Yu Hakusho and Hunter x Hunter, and Naoko Takeuchi, creator of Sailor Moon, have been married since 1999, so a connection between the series isn't the least bit far-fetched. In fact, the two have collaborated before—Takeuchi helped Togashi add screen tones to his work, and the two created a children's book together.
There has been some tangible crossover in the couple's works, like when Kurama from Yu Yu Hakusho claims to be Tuxedo Mask as he tosses a rose. But could Tuxedo Mask be a real person in the world of Yu Yu Hakusho?
The characters in Yu Yu Hakusho spend a whole lot of time in the underworld beating up demons while the Sailor Scouts are on Earth or in space. Each group could be taking on enemies in different realms.