Historically, Ni no Kuni has taken an almost Final Fantasy-style approach to sequels. Ni no Kuni: Wrath of the White Witch and its only-in-Japan counterparts, Dominion of the Dark Djinn for DS and Hotroit Stories for mobile devices, exist as one story. The console sequel, Ni no Kuni II: Revenant Kingdom, takes place many generations later, with any ties to the original basically functioning as fun Easter eggs. Characters and plot don't move between games, but themes, the art style, and certain enemies and races do. The 2019 Ni no Kuni movie took a similar approach and did not feature direct overlap with the games.
The Ghibli inspirations remain strong in Ni no Kuni: Cross Worlds.
The upcoming No no Kuni mobile MMO, Cross Worlds, is continuing the tradition and not relying on the story that came before it. But there are plenty of reasons to recognize Cross Worlds as a legitiamte Ni no Kuni game that understands its roots--and it might be more of proper sequel than assumed.
The original Ni no Kuni tells the story of a young boy who loses his mother at a young age and learns of a fantasy world in need of his help. He enters that world in order to save it and maybe learn a little bit about himself along the way.
