The most disastrous sequel to an anime I have ever seen is Tokyo Ghoul:re. It completely ruined the original manga's story, characters, and themes. The animation was choppy, the pacing was rushed, the dialogue was bland, and the plot was full of holes and inconsistencies. It felt like a cheap cash grab that disrespected the fans and the source material. I was so disappointed and angry that I dropped it after a few episodes. It is a shame that such a masterpiece of a manga was adapted so poorly. I wish they would remake it with a better studio and more episodes.