PVP Coliseum

[Felis]

Well, I presume they'd just be using the existing Coliseum under the GLA Guild in Ul'dah, which is already an instanced area (it's only used for a couple of PvE fights related to the GLA class quests), so it wouldn't take a lot of alterations/reprogramming to change it into a PvP arena.)

Only other thing I can think of is this hypothetical 'Gold Saucer'-like area that has been mentioned as a possibility for the game might include something similar (Gold Saucer in FFVII itself had a battle arena after all).

Dev team make a new building in Limsa as PvP arena (Limsa need more touristic spots ^^).