There are a lot of assholish nations that could easily take that place, but imo there are enough references to Dalai lama in the show to assume that the fire nation is china
Being "just a cartoon" I never thought of the dimension of it, but they literally wiped out an entire nation. Suddenly, comparing the fire nation to any current superpower seems wrong.
The earth nation exists with a whole theme about being the great stagnant central state empire getting overshadowed by an upstart nation going through industrial revolution first doesn't give you any hints?
Not even the queues and qibao?