Hayao Miyazaki has retired from feature filmmaking. But, before he did, there was a manga he was keen to turn into a movie, but didn't. That manga was Parasyte.
In a recent podcast, Studio Ghibli co-founder Toshio Suzuki talked with Genki Kawamura, who produced the live-action Parasyte movies. Suzuki told him, "We were thinking about doing [an anime] of it at Ghibli."
Somewhat surprised, Kawamura said the manga's themes did match up with Studio Ghibli's sensibilities. Hayao Miyazaki, in particular, apparently liked the manga. According to Suzuki, "I also really liked it... The original manga was very interesting." The two of them moved forward with the project; however, they obviously ended up not doing the anime.
This was a while back, and at this time, the movie rights were held by Hollywood. It wasn't until the year before last that the rights returned to Japan, which explains the recent glut of Parasyte adaptations, including a not-so-good live-action version.
Kawamura pointed out that elements of Parasyte were reminiscent of Nausicaä of the Valley of the Wind -- something that Suzuki agreed with.
Anime studio Madhouse is currently doing an animated Parasyte TV series. It's been popular with fans, but damn, who wouldn't want to see Miyazaki's take on Parasyte? Instead, we got Ponyo.