The possibility seems remote, but imagine, Dear Reader, that find yourself playing a game of Night-Haunted Hogwarts. Further imagine that none of the 24 clan options suits your purposes - perhaps you have a favorite character that I failed to include (cutting it down to a d6 roll's worth for each house left many on the table), you don't like my interpretation of one of the existing ones, or you really want to play something off the beaten path like a foreign wizard, goblin, centaur, or elf. What to do?
In this post I'll walk through my process and see if we can distill that into some general principles that we can use as a guide for making your own material in this setting.
As I've discovered, writing tie-in material for an established setting you have to fill in a lot of blanks left by the text. A roleplaying game needs different things than a series of boarding school mystery novels does, for example. At the same time, your work needs to feel like it belongs or you lose the benefits of using the established setting in the first place. Writing tie-in material, therefore, is mostly a process of extrapolation - you're trying to surprise your audience with a sense of familiarity. In practice this mostly consists of looking at specific examples from the text and expanding outward to make a generalized type. These can be direct (Neville Longbottom killed one specific snake one time --> his descendants are famous dragonslayers and serpent-hunters) or indirect (a mirror in the Leaky Cauldron gives Harry sarcastic advice on his appearance --> mundane objects in Harry Potter world can have funny personalities that relate in some way to what they do --> the Crabbe's magical caltrops chortle to themselves whenever they think they're about to stick someone). Sometimes they are made from the thinnest of thin strands (house-elf could be said to imply other kinds of elf: by looking at house-elves what can we interpret as qualities of "elfishness" in Harry Potter and how can we apply them to different contexts).
