Imo the most sense would make ice boundary line created by Kyurem. Not frozen world, not blizzard, just boundary line separating Unova. In R/S/E in Rusturf Tunnel young couple is separated until player destroys boundary rocks. Similar thing could be done with ice boundary line. I think we'll be able to see people from other side trying to break ice boundary line with no success until Kyurem is caught by player.