When Osamu Tezuka died, he left his life's work Phoenix unfinished. Which is an absolute shame, as he quite clearly had great plans for it. Had he stayed alive to finish it, the separate stories of each time period in each volume would eventually converge at a central point, the "present", and tying all the loose ends of the Phoenix's story together.
The 10th and last volume of Barefoot Gen ends with the words "End of Part 1". Keiji Nakazawa had always intended to write a "Part 2" that would chronicle Gen's life in Tokyo, but he was forced to scrap these plans for good in 2009 due to developing cataracts and vision problems. He died of cancer just three years later.
Noboru Yamaguchi tried to avert this trope by attempting to finish The Familiar of Zero as fast as possible while also undergoing treatment for cancer. He died on April 4, 2013. That said, the series was completed anyway between 2016 and 2017 via two additional volumes, based on Yamaguchi's notes.
Ken Ishikawa, the creator of Getter Robo. Though he did leave some notes behind, they were apparently only for Getter Robo Hien - a prequel. The cliffhanger ending of Getter Robo Āḥ, the latest series chronologically, has yet to be resolved.
The director of Mobile Suit Gundam: The 08th MS Team, Takeyuki Kanda, died while the OVA was still being produced. One effect of that was that it took over three years to finish it, making it the longest-running single Gundam production until Gundam Unicorn, except the long-running period for that has been planned from the start.
And then, a decade and a half later, the guy who took over for him died while working on Towa no Quon.
Guyver fans also fear this with Yoshiki Takaya. We don't know much about his habits and health but since he's been doing his great work since the 80s, works a monthly manga, has no assistants and takes breaks, we fear that he may cease to be before the manga ends. Up until a few months ago, some thought the manga was beginning to wind up for a climax when a new thread appeared that could be sign of the end being a bit further away than we thought.
Trinity Blood's author Sunao Yoshida died of lung blockage in 2004 and the story was finished sometime later by other writers.
Kaoru Tada's manga Itazura na Kiss was left incomplete when she passed away after a tragic accident. Luckily, she had the ending semi-planned out, and it was used for the anime.
Perhaps not a total Author Existence Failure, but what happened to mangaka Kyoko Okazaki certainly comes close - in 1996, she was hit by a drunk driver before completing the last few chapters of her Body Horror-esque cosmetic surgery drama Helter Skelter. The resulting accident left her quadriplegic and mute. The manga was finished in 2004, by her former assistant Moyoco Anno (yup, the wife of Hideaki Anno), working from Okazaki's storyboards and with her consultation, albeit on a Cliff Hanger. Poor Mrs. Okazaki's rehabilitation continues to this day - more than 20 YEARS later.
Yoshito Usui. Tragically killed in a hiking accident in September 2009.
On the other hand, Crayon Shin-chan is more like a newspaper comic in terms of presentation (i.e., there's a "sliding timeline", and doesn't really have much of an overall plot; the anime is more like a Sitcom in this respect), so his death won't have that much of an impact on the original manga. The anime (which mostly uses original stories not found in the manga anyway) was put on hiatus for a few weeks after his death, but it was later announced it would continue. Now the word has come that (in another parallel with newspaper comics) the manga will also continue with a new author.
Keiko Tobe, the mangaka of With the Light, died of an unspecified illness in late January 2010, leaving the manga unfinished.
Satoshi Kon suddenly died while making a film called The Dream Machine, which he described as aimed at a younger audience but accessible for his older fans, with a cast made entirely of robots.
Kon learned that he had late-stage pancreatic cancer in April 2010, but initially didn't tell anyone because he didn't want to disturb them — cancer is still something of a taboo in Japan. He later regretted this decision and verbosely apologized to everyone in his farewell note (he ended it with an apology for "leaving while everyone else was still working") after his producer Masao Maruyama promised to him to finish the movie.
MM!'s author Akinari Matsuo passed away without warning on April 18, 2011. The phrasing of the letter, which does not mention accident nor illness, suggests suicide.
The 2019 massacre-and-arson incident that hit the Kyoto Animation studio impacted several projects like the second Violet Evergarden movie (finished by another team), the second season of Miss Kobayashi's Dragon Maid (delayed until 2021) and a new Free! movie.
Often happens in regards to voice actors:
Yasuo Yamada, most recognizable as the voice of Lupin III, died of a brain haemorrhage in 1995, a month before the release of Farewell to Nostradamus, the movie that would mark the series' return to the big screen in ten years. Fortunately, his will stated that, if he were to pass away, the baton should be passed to Kan'ichi Kurita (a popular comedian and impersonator), who currently voices him from that movie onwards. In honor of Yamada, a message was placed at the end of the film's credits: "To Yasuo Yamada, Eternal Lupin the Third: Thank you!".
Miyu Matsuki, AKA Choppy from Futari wa Pretty Cure Splash Star, died from a viral infection right before the release of the Pretty Cure crossover movies. She managed to record her lines as Chihaya from Persona 5, but was replaced by Haruka Terui for the anime and Royal.
In the case of the Inuyasha sequel Yashahime: Half-Demon Princess, Miroku's seiyuu Kouji Tsujitani died before its release and his English VA Kirby Morrow passed away after working on the first episode. Makoto Yasumura took over the role in Japan, and Ian James Corlett did the same in the West.
Some of the original seiyuu for the Ranma ½ anime died before the 2024 remake was announced: Hirotaka Suzuoki (Tatewaki Kuno), the aforementioned Kouji Tsujitani (also Kuno, plus Ranma's school friend Hiroshi), Hiromi Tsuru (Ukyo) and Ichiro Nagai (Happosai). As of September / October 2024, Tomokazu Sugita has taken over as Kuno for the first cour.
Played straight with the Sailor Moon Cosmos movie's Latino Spanish dub: the first anime's VA for Sailor Galaxia, Nancy MacKenzie, lost her battle with cancer in June 2024. While she did voice Galaxia in the trailers, the movies proper has Julieta Rojas replacing her.
Tadao Nagahama, the director behind the RobotRomanceTrilogy and many other old-school series, died of hepatitis in 1980 while he was working on the Rose of Versailles anime (and he already had huge Creative Differences with Oscar's seiyuu Reiko Tajima, who was mostly a stage actress and had problems with following his directing style). He was succeeded by Osamu Dezaki.
Akira Toriyama was working closely with the production of a new Dragon Ball anime, Dragon Ball Daima (set to be released in Fall 2024), especially in regards to the story and character designs. An anime movie adaptation of his 2000 manga Sand Land had been released in 2023, and an ONA (Original Net Animation) was set to be released on March 20, 2024. Tragically, Toriyama passed away following complications from brain surgery on March 1, 2024, less than three weeks before the aforementioned ONA premiered.
