Japan would be a great locaton for FH5 but for the element of surprise, Canada or Dubai would be very interesting.
Canada and Dubai I can actually see as strong candidates now that you mention. I really want to think Hawaii but since test drive did that a decade ago I just don't think T10 would venture to that, at least not yet. But definitely Dubai especially since the track is in Forza 7 and like you said the car culture is strong and plus it's beautiful there.Uhhhh....let's see.
Australia/Oceania is done, we aren't coming back to it (at least for the time being)
FH4 is set in a country in Europe, and I doubt we would get two Europe games in a row
My gut feeling says South America/Latin America/Africa are out of the equation.
So here are the places I see that are most possible:
Dubai (a lot of car culture, sand dunes everywhere)
Japan (a lot of car culture, switchback mountain roads, and potentially city street races)
Return to Colorado (an expanded version of the FH1 map)
Hawaii (a lot of verticality, varying landscape, and maybe they can do a 1:1 representation of one or more islands)
Canada (would work well with the seasons mechanic)
Various streets and highways that, when connected together in the right way, faithfully recreate the original Fuji Speedway including high banking, and Suzuka complete with the original 130R corner and all the original layout.Apart from the Obvious Touge roads it needs the Highway Loops and possibly even a real track hidden In the map, like Tsukuba.
They could put some Lego trees here and there.it doesn't change the fact that it's sand as far as the eye can see with almost no flora
after one month everyone will be tired of it, there's no way that they choose Dubai/UAE
I can't see why the scenery is limited to real word. It's not that any Horizon map represent real word.