Series that started out good but got indefinitely worse
Sometimes tv series/anime/manga etc. start out pretty strong with much more story to tell. But for some reason, the quality drops so hard that they just vanish from pop culture, with many of the later seasons not being mentioned ever again. Spoilers ahead of course.
Edit: Title should say "infinitely worse"
- Komi can't communicate
Over the course of several hundred chapters readers learned to love the quirky characters of this manga (except for one...) and the slow but sweet progression of Komis and Tadanos relationship. But nearing the end of the manga, the author has the brilliant idea to reintroduce Tadanos first love Rami Kawai, who becomes a fan hated staple and third wheel to the relationship for the rest of the manga, even living together with Komi in the end.
- The promised Neverland
While the manga had its own problems, it was a tight story with interesting characters that grew and developed over time and an incredibly thrilling atmosphere. The anime however, after a strong first season, decided to skip most of the manga and basically go directly to the final arc with an alternative ending. The probably most beloved arc, Goldy Pond, was completely skipped while being the perfect showcase of what made this manga great.
- Heroes
While Heroes does not have such a sudden drop in quality as the first two examples, it still wasted so much potential. After season one everything went slowly, but steadily downhill. Somehow, seemingly dead characters are alive again. Powercreep happens a lot just for the solution to be amnesia or mysterious illnesses. The cast also grew bigger and bigger with the story becoming thinner and thinner at the same time.