Usually, a novel has 2 storylines.
One is the adventure, and the second is a story of a personal change.
With the adventure, I would look at your episodes, and look for a pattern. In izekai usually they want to go home, defeat Ancient Evil or achieve immortality.
With the personal change, that’s the life lesson the hero learns on their journey, like accepting responsibility pr learning to love.
One of the best books you can read is Save the Cat, Writes a Novel. It is really helpful for looking at genre, structure and goals of a story