I don't really want to see open world PvP, but having some engaging PvP offerings could create a vibrant community within FFXIV itself.
Going up against other players is the only true test of skill since you'll be up against intelligent opponents with unexpected tactics, unlike your typical, scripted PvE fights where the routine is completely predictable. And online fighting games are immensely popular just for the thrill of the combat itself, even without the need for special rewards. PvP has huge replay value, even with a minimum of updates, that keep people entertained for ages.
Really, the best kind of question that needs answering is why did FFXI's Ballista, Brenner, and even Pankration never really gain any momentum after they were introduced? They were popular for a time, but seemed to fade out, almost tragically. If you could solve that problem, then PvP would be a welcome addition to the game, I'm sure.
