That's kinda crazy to me. There are 6 other reflections of our world that can each be hugely unique and different to our own, and you think they should just ignore them all? In that case they may as well ignore all the other continents in eorzea too because we went to another continent in stormblood so doing it again with a different continent would be dull and redundant. At this point they literally can't do anything completely new in premise - we've had travelling the planet, betrayal, wars, killing god, travelling the universe, travelling to different dimensions and travelling through time- that covers basically all rough plot points available lol. So all they can do now is one of the above, but in a different way and with different plots and lore. Whether that be a new continent, a new war or a new shard is up to square, but none of it would be boring unless they're lazy or unimaginative about it.