How would you determine who is an ally and who is an enemy? There are no playable factions in the game that are in opposition to each other. If you were thinking free for all, you're going to have a very hard time getting players to opt in and most of those who do are going to be more interested in hit-and-run tactics against lower level players than fair fights.

This game just isn't designed for world PvP and trying to introduce it at this point wouldn't sit well with the majority of the player base because even if you're not directly involved in the fighting, it's usually disruptive to other reasons for being in the zone. Create special zones just for it and it stops being world PvP.

Better to get the world PvP in a MMO that is designed for it if you can find one where it still thrives.