Gonna throw in another guess for where Autumn Ring might be set.
The benefit Polyphony has with its fictional circuits is that they have the ability to set them anywhere. Look at how High Speed Ring was put in a very remote part of Japan's north island Hokkaido for example.
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What I also notice on the Asia-Oceania map in World Circuits is that while Japan itself is densely populated with track icons, the west of the map is less so, particularly in the area of Korea which takes up a decent chunk of the space.
So Polyphony might choose to set Autumn Ring somewhere in South Korea, rather than Japan.
We recently saw from screenshots of the Spec IV intro movie that Autobacs is a notable sponsor at Autumn Ring, similar to how WeatherTech is at Grand Valley, so Asia would be the best guess for the continent. And although Autobacs don't have any stores in South Korea, they did still feature on the GT4 circuit Seoul Central.
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So that's a new theory/guess.