There is no "homebrew fluff" yet - at least none that I know of. Hence, only the rules would be affected at this point.
The only official thing about Nippon in the Ninth Age so far is that there are islands shown on the world map East of Tsuandan. Nothing else has been created about it, ever, be it in homebrew or by the official teams.
So anything you would write could be used as a brainstorming for the project at a later stage.
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GHAÂAÂAÂARN ! — The Black Goat of the Woods with a Thousand YoungFirst T9A player in West Africa