Five Nights at Freddy's -- she's watched one of her favourite YouTubers play the games repeatedly, and then we got the trilogy of tie-in novels from the library. She finished the third book just last night and was absolutely convinced there had to be a 4th book, since she thought too many questions were left unanswered. It was after I Googled enough to be certain that the 3rd was the end that she started musing about fanfic.

(She wrote another few pages this morning, and is in fact probably writing now in bed, I should go remind her to turn off her light!)