I wouldn't be opposed to it if it was going to be popular among a decent number of players. I do see it as difficult to implement within the framework of the narrative. All PvP in FF14 is effectively Wargames between the GCs and not to the death. Infact the whole point of the Frontlines is effectively creating a form of tournament to decide issues to avoid military tension and conflict between the City States.

I've also very rarely seen open world PvP work outside games revolving around PvP. The core issue is it tends to supress PvE activities like gathering because of the interference involved and usually ends up just being high level people going around ganking lower level ones. It gets worse if the factions aren't balanced in player numbers, potentially making areas of the game impossible to play for players of a weaker faction.