I'd have nothing against this feature if it was already in the game.
But I would prefer the developers spend their time creating new content instead.

And I don't think this feature is as simple as people think.

For example, have you ever been doing a quest that spawned monsters, and had someone else spawn the monsters at the same time? Should those monsters the other person spawned (which except for levequests are targetable by anyone) show up under this option? Should they be able to target (and kill) the monsters you spawn?

And if you can't see other players, can they see you?
Can they target you, and cast spells on you? Do you click-block them?

Are you allowed to participate in Fates or Hunts while hidden?
I can just imagine the griefing an invisible tank could pull if that were allowed ...

So there's a lot more going on here than just hiding other people in gpose.
And because of that, I don't want hide-all-other-players added to this MMO.