Most delayed sequels ever
Anime
Sequels to animes that took years to release. This list is informative and is not intended to be critical of these anime.
Thanks to the users eblf2013 and Andrewapps for collaborating.
Thanks to the users eblf2013 and Andrewapps for collaborating.

TV, 1992,
47 eps
Me:-
Author:-
Tetsujin 28-gou -> Chou Dendou Robo Tetsujin 28-gou FX 28 years.

Movie, 2024,
1 ep
Me:-
Author:-
Kidou Senshi Gundam SEED Destiny Final Plus: Erabareta Mirai -> Kidou Senshi Gundam SEED Freedom 18 years.

OVA, 2017,
1 ep
Me:-
Author:3
Cardcaptor Sakura Movie 2: Fuuin Sareta Card
-> Cardcaptor Sakura: Clear Card-hen Prologue - Sakura to Futatsu no Kuma 17 years.
-> Cardcaptor Sakura: Clear Card-hen Prologue - Sakura to Futatsu no Kuma 17 years.

TV, 1980,
51 eps
Me:-
Author:-
Tetsujin 28-gou -> Tetsujin 28-gou (1980) 16 years.

OVA, 2004,
6 eps
Me:-
Author:-
Top wo Nerae! Gunbuster -> Top wo Nerae 2! Diebuster 15 years.

OVA, 2002,
13 eps
Me:-
Author:-
Saint Seiya -> Saint Seiya: Meiou Hades Juuni Kyuu-hen 15 years.

OVA, 1998,
1 ep
Me:-
Author:-
Golgo 13: The Professional -> Golgo 13: Queen Bee 14 years.

Special, 2009,
1 ep
Me:-
Author:-
Yuu☆Yuu☆Hakusho -> Yuu☆Yuu☆Hakusho Picture Drama 14 years.

Movie, 2015,
1 ep
Me:-
Author:-
Digimon Adventure 02: Diablomon no Gyakushuu -> Digimon Adventure tri. 1: Saikai 14 years.

Movie, 2000,
1 ep
Me:-
Author:5
Vampire Hunter D -> Vampire Hunter D (2000) 14 years.

ONA, 2020,
12 eps
Me:-
Author:4
Koukaku Kidoutai: Stand Alone Complex - Solid State Society -> Koukaku Kidoutai: SAC_2045 13 years.

ONA, 2024,
5 eps
Me:-
Author:-
Kimi ni Todoke 2nd Season -> Kimi ni Todoke 3rd Season 13 years.

-, 2025,
? eps
Me:-
Author:-
Panty & Stocking with Garterbelt -> New Panty & Stocking with Garterbelt +12 years.

ONA, 2022,
6 eps
Me:-
Author:7
Yojouhan Shinwa Taikei -> Yojouhan Time Machine Blues 12 years

TV, 2018,
24 eps
Me:-
Author:-
Basilisk: Kouga Ninpou Chou -> Basilisk: Ouka Ninpouchou 12 years.

TV, 2020,
24 eps
Me:-
Author:-
Higurashi no Naku Koro ni Rei -> Higurashi no Naku Koro ni Gou 11 years.

Movie, 2025,
1 ep
Me:-
Author:-
Mahou Shoujo Madoka★Magica Movie 3: Hangyaku no Monogatari -> Mahou Shoujo Madoka★Magica Movie 4: Walpurgis no Kaiten 11 years.

TV, 2020,
24 eps
Me:-
Author:3
InuYasha: Kanketsu-hen -> Hanyou no Yashahime: Sengoku Otogizoushi 10 years.

TV, 2022,
13 eps
Me:-
Author:-
Bleach -> Bleach: Sennen Kessen-hen 10 years.

TV, 2022,
12 eps
Me:-
Author:-
Hataraku Maou-sama! -> Hataraku Maou-sama! season 2 9 years.

Movie, 2021,
1 ep
Me:-
Author:-
Evangelion: 3.0 You Can (Not) Redo -> Evangelion: 3.0+1.0 Thrice Upon a Time
8 years.
8 years.

OVA, 1992,
6 eps
Me:-
Author:-
Macross: Do You Remember Love? -> Macross II: Lovers Again 7 years.

TV, 2018,
26 eps
Me:-
Author:-
Toaru Majutsu no Index II -> Toaru Majutsu no Index III 7 years.

TV, 2024,
11 eps
Me:-
Author:-
Kuroshitsuji Movie: Book of the Atlantic -> Kuroshitsuji (Shin Series) 7 years.

TV, 2024,
12 eps
Me:-
Author:-
Ao no Exorcist: Kyoto Fujouou-hen -> Ao no Exorcist (Shin Series) 7 years.

TV, 2024,
12 eps
Me:-
Author:2
Sword Art Online Alternative: Gun Gale Online -> Sword Art Online Alternative: Gun Gale Online II 6 years.

TV, 2020,
25 eps
Me:-
Author:-
Toaru Kagaku no Railgun S -> Toaru Kagaku no Railgun T 6 years.

TV, 2024,
25 eps
Me:-
Author:-
Fairy Tail: Final Series -> Fairy Tail: 100 Years Quest 6 years.