My opinion on all of the FH6 location speculation is that it doesn't really matter a ton where it's set, in terms of map diversity and car culture. The maps are a handful of national landmarks scattered into a totally made-up place which they can do anything with, and the full extent of Mexican car culture in FH5 is a single story where the same old Beetle model that's been around for years gets called a Vocho in dialogue. Eventually we got some mentions of the Baja and a Tsuru too, but 95% of the narratives and characters could be anywhere in the world, just with different accents.
To me, Japan is the obvious 'expected' setting as it was for 5, and China is another obvious pick with 5's introduction of their cars. I think the Finland suggestion earlier is fun, but I personally still want to see it set in Africa, because we need more games and other media set outside of western Europe, North America, and Japan. Let's see Kenya, or Thailand, or Estonia, or somewhere else interesting. Perhaps Horizon India would be a fun setting at some point too.