The recent survey mentions a number of real-world locations including Peru and Alaska, as well as historical settings like the Vietnam War and an Old West-styled 19th century America.
But there are a few stranger possibilities too, like "a Far Cry game where you can fight against or join vampires," "a Mad Max-style post-apocalyptic world" and "a Far Cry game set in the modern day on a Jurassic Park-style island with dinosaurs." Of course, there's also mention of Far Cry with zombies.
Franchise developer Ubisoft has experimented with quite a few settings throughout the series. Far Cry 4 saw players dropped off on tropical islands and the Himalayas, while Far Cry 3: Blood Dragon played with a 1980's styled neon landscape and Far Cry Instincts and Predator pitted users against genetic experiments.