Hogwarts is 7 years, 4 houses, 30 students per cohort, and endless combinations of pairings, friendships, and upheavals. The canon leaves gaps a decade wide: what was Snape doing during the First War? How did Remus survive between full moons? What were Lily and James thinking in their seventh year? Natural territory for AI co-authoring: test an idea fast, see if it holds in a literary register.
Name the era (Marauders, Trio, Next Gen, Pre-Hogwarts, Cursed Child), the house, the leads, and the tone. The AI returns a chapter with the canon's gothic-academic atmosphere: corridors, portraits, the Forbidden Forest, the murmur of the Great Hall. In English — registered to the British literary tradition, without machine artifacts. Stop the stream, swap a scene, pick up the saga a week later.