Give the players the option to attack or being attacked in the open world. Without this option activated the players can stay neutral and not being able to attack or get attacked in open world. This option will give more liveliness to the game... I mean in WoW works, so why not here. By putting the tick ON you are agreeing that you wont mind being attacked by any kind of player of any kind of lvl, class, build etc. With a simple tick in the character select menu you can decide which character can be aggressive and which normal/neutral.
Yes, we have WvW and PvP, but those are zones that are just for fighting players and nothing more... the same old battlegrounds with no element of surprise where you can't go lvl up, kill some champion, world boss and being ganged at any point.
We observed this kind of implementation in the game where from neutral you go attackable in the small arena in the PvP lobby, so we know the mechanic is there but used on a very small scale.
Just think about the possibilities, dueling, GvG, bounty hunting players and many more on all kinds of terrain. Making creative combat videos with cool surroundings, using proper arenas and getting huge crowds to watch community driven tournaments while being save and bidding for the winner. The possibilities are limitless and in my opinion the devs can implement this in the game quite easy... I mean look what they've done with the stats change menu, it's f***ing gorgeous! One simple click on the character swap menu making you being able to fight with other players who clicked the same option and the open world, this game will never be the same again!