Having played FFXI and left it behind since ARR dropped, I think a Dynamis-type concept wouldn't be beyond the pale in Eorzea provided a suitable storyline were provided to justify it's existence.
Example: What's to stop the Ascians from recreating our world in their own warped image, with Beast Tribes replacing us as inhabitants of each city-state? Could you imagine Limsa Lominsa run by Sahagin and Kobolds? Sylphs & Ixal in Gridania? Amalj'aa in Ul'dah? Let's take it beyond that: Gigas in Coerthas or Gnath/Vanu in Ishgard.