Maybe you can go to a certain NPC or do certain actions to get off your World PvP flag, I really don't like the whole simplicity of doing everything with just a simple click.
The other thing that could be done is making a max level difference, if I'm level 20 and you are 25 we can fight, but if you are level 35 you can't fight me no matter I have world PvP enabled.
The zones could also be scallable so that high level players are scaled to a lower level if they are in a lower level zone. This will make PvP balanced and low level players will not be killed by higher level ones. The only issue with this is even though higher level players will be scaled to a lower level they will still most likely have more spells than the lower levels since their actual level is higher. That doesn't mean that the lower levels won't have a win condition, but they will just need to use the terrain and any advantage they can think of to outplay the scaled higher level.
~ Yours Truly