I've been working on thinking about characters that might be in this book, in addition to Seikei and the Judge. The original thought I had was that Seikei would be entrusted with the care of a Dutch boy about the same age. I'm thinking that the Dutch boy won't speak Japanese, and Seikei, of course, won't speak Dutch.
First thought I had was that the boy might be the son of a ship's captain. They are guests of the shogun, who did frequently like to see the Europeans, more as curiosities than anything else.
Conflict comes when the captain is killed--but why?
[If I don't have a solution to that problem, I go right on jotting down notes. Usually I find the answer eventually.]
Are there other non-Japanese guests? One of them might kill the captain because he wants to take over the lucrative trade with Japan. Maybe a Chinese, who also came into the port at Nagasaki.
A death in the Shogun's palace is serious, a blow to his honor if a guest is killed there. Death will have to come by poison, or some other method where the killer is not immediately obvious.
So the Judge is called on to solve the case. Maybe he and Seikei are already there, to see the foreigners, whom Seikei has heard are "red-haired devils." He's surprised when they don't have red hair.
Maybe one of the things the foreigners are displaying to the Shogun is Western-style food. The poison comes in that.
The judge cannot hold the other foreigners for long, so he entrusts the captain's son to Seikei, who will escort him back to Nagasaki. Maybe they leave a day or two before the others. The Judge wants to see who will pursue them?
Have to find a route that a traveler took through Japan. [That was Shiba Kokan, the scientist and artist that some call the Leonardo of Japan.]
Along the way, Seikei and the boy will meet interesting people, some helpful, some dangerous. Like to work in a Zen monk. He shelters them along the way? Helps them? How?
Anyway, this shows you how starting to think of characters helps you to form a plot. It works for me. Maybe you'll find a better way that suits you. Kurt Vonnegut once told me he had big charts on the wall where he tried to outline plots before he began to write. But he admitted that he hardly ever finished the story with the plot he had at the beginning.