Aht Urghan Whitegate has multiple sets of arena doors for a pvP system that was never implimented. I think that, should they add a PvP system, it would have to include some kind of gear reward system as an incentive to do that over PvE. Perhaps players could accumulate points toward PvP-exclusive gear. This gear could rival certain piece of high-end gear currently available. SE could also make the points redeemable for random AF 3 seals.

Arena battles could range from 1-vs-1, 6-vs-6, and alliance; one-shot and round robbin tournament play. They could also reinstate the cross-server PvP match ups they used to hold for top teams and soloists across every server. This would reduce congestion in Abyssea, probably.