Well, never say never … but at present it seems unlikely, for three reasons. Firstly, Hayao Miyazaki has spent more than a decade working on two films. The Wind Rises and The Boy and the Heron, that are deeply personal. They are explorations of his own ideas, inspirations and beliefs, and of events and relationships that have preoccupied him since childhood.
He used several other authors’ works as springboards for these two films, but they are not faithful or even close adaptations of the original. For him to return to adapting the work of another author, I think it would have to offer him the chance to explore something new, but equally personal, something that could be tied into his own experience. He hasn’t been moved to adapt the other two Ingary books since 2004, when Howl’s Moving Castle was released; there’s no obvious reason why he would do so now.
Secondly, Studio Ghibli has only one other director at present, and Goro Miyazaki has shown no interest in making sequels to his father’s films. So a new young director would need to be found, hired, and allowed to finish the work on his (or just possibly her, though Ghibli’s track record on women in leadership roles isn’t strong) own terms. The last young director to adapt a Diana Wynne Jones book at Ghibli was Mamoru Hosoda; his version of Howl’s Moving Castle was abandoned in 2001, and Miyazaki Hayao took over. That might not happen again, but under its new ownership at NTV, Ghibli seems in no hurry to hire new young directors.
Thirdly, Hayao Miyazaki is now 84 years old. He is said to be working on another new film, and it’s possible he may live to finish it – after all, live-action director Yoji Yamada is 94 years old and his 91st film, Tokyo Taxi, was shown at this year’s Tokyo International Film Festival. But animation is hugely labour-intensive, and while Miyazaki’s legendary work ethic is still intact, his work has slowed down. Even with the full support of a stellar team and all the resources of studio Ghibli it took seven years to make The Boy and the Heron. So, realistically, it’s possible we may get one more animated feature directed by Hayao Miyazaki, but it seems unlikely that we’ll get two.