Open world PvP in FFXIV would have to be Very limited.

The examples I would look to from WoW that could be examples for limited implementations would be perhaps Gurubashi Arena, or the Dalaran Sewers. Both certainly had their flaws, but there was a lot of fun that came out of it, and it was completely optional content. A simple moshpit where any who enter are free to be fought, preferably with the caveat of light/full parties being disallowed to enter.