I think it’s ok to offer that as a prompt (as long as it is still mainly and clearly about your fandom, not big fandom) but I would make sure to include other options. On a purely subjective level, I think it’s better if it’s a fusion (i.e. set at Hogwarts), rather than “what if X characters from my fandom are on the run from Voldemort and cross paths with Harry, Hermione and Ron?”, but that’s a preference, not Yuletide gospel.