It seems if you have unexplored regions (of high mountains) then Frost caves can appear in those.. so if they dont force us all to restart when they eventually leave early access.. it might be wiser to not explore widely... ie Oceans no real biome details there.. ASshlands same.. maybe plains might get something else??.. "Mistlands?.. well that could be converted BF maybe.. its still a work in progress but for now frost caves should appear in mountains!