Man remember how interesting it was to fight Bismarck on an irregular turd-shaped rock? No?
There's some design room for like, pillars you hide behind and the like in some fights, sure, and even though it is ultimately a box, the Hippokampus fight has long phases where you're functionally restricted to a very limited track-- but ultimately there's only yea so much you can do with arena design other than add visual clutter to distract from the fight. They've tried stuff like the bridge in Ishgard with the towers and people largely hated it.