If you simply must have open world, why not just add the Cartenau Flats as an option to travel to? It sits above Northern Thanalan, but below Coerthan areas (side note: where exactly is Seal Rock or the Fields of Glory?). If people want an open battleground, let em have it with that. Adjust/add fates and hunts as more of a PvPvE thing, maybe some Garlean attack teams if need be. Enforce a minimum level/item level if need be, no DoH/DoL, no nodes, warning/confirmation upon selection to travel to it.

Or how about even reviving the Diadem that way? A special "Sky Pirate Free-for-All" mode (but no LITTLEFOOT).