CROSSOVER MADNESS

 Most manga serials didn't delve into any crossovers, but GHOST SWEEPER MIKAMI (1991) crossed over the main character, Japanese exorcist Reiko Mikami, with the folkloric character of THE PIED PIPER OF HAMELIN.

 To be sure, it's really just a minor demon that tosses together a lot of tropes from the Pied Piper story and reworks them for its own convenience (much like the manga author), such as changing adult humans into children or controlling rat demons. But one could argue that it's still a loose adaptation of the original, though it counts only as a "high-charisma" crossover.

The manga got an adaptation into an episode of the 1993 TV anime, under the title "Everyone, Turn Into a Kid!" Again, the name "Piper of Hamelin" just gets cursory mention. 

Other manga-episodes, as well as their anime adaptations, brought the ghost-sweepers in contact with Santa Claus and with the Japanese folklore-figure Yuki-Onna. The one movie made from the franchise was not based on a manga from the author and has the GS team encounter a very Nippon-ified version of the vampire Nosferatu.