I wouldn't mind a PK server, but I think it would be fairly unbalanced considering the way each class / job works in the game.

That being said, in other games I've played with PK, there is usually some penalty for PKing someone. The more you PK, the weaker you become and your name visually changes color or something. You know, to make you a target for other players.

One thing I did like about PK was that you get summon your friends to protect you when you want to go do something. There is a sense of apprehension when you go out into the world. A sense of danger. Now being level 60, almost nothing in the game gives me a sense of danger. Very few things keep me on my toes and alert.
But not knowing if another player is hiding behind a corner, waiting to ambush you, keeps you alert.

IF there was to be PK in this game, gathering / crafting classes would have to be excluded.

But, what if they were to implement a PK zone? NOT like Frontlines, where you have to 30 minutes or more to get in and complete an objective. Also, there are really no places to ambush people in Frontlines. Enemy movement is too predictable.
Maybe by playing in the PK zone and killing players, players can receive some points or something (like wolf marks) that could be exchanged for items. Items which are NOT exclusive to PvP, but can also be obtained in other ways, should a player not choose to PvP.

I wonder if something like that would be possible.