I assume Horizon 6 is already planned and/or well into development, so consider this a suggestion for Forza Horizon 7. (Unless I’ve accidentally hit on what you’re doing next.)
So this location would extend from, say, Portland Oregon, up through Seattle, past Vancouver, and include Vancouver Island and Whistler.
This has a mix of terrain, from cities like Portland/Seattle/Vancouver, beaches at Tofino, mountains at Whistler. Inland forests, rainforests, rivers. Ferry to Vancouver Island. Lots of two-lane roads, but there are interstate highways for those who want that, at least on the American side.
The Pacific Northwest has lots of great locations (including Redmond WA, so you can include Xbox HQ), and the Sea-to-Sky Highway to get up to Whistler is an iconic drive. (The old Ski-and-D*e Highway, before it was widened for the 2010 Winter Olympics.)
It’s time for Horizon to return to the USA, and “Cascadia” is the ideal place to put it. A border-straddling region like that of Forza Horizon 2 is my firm suggestion for a future Horizon festival.