Aion achieved what you are looking for.. VERY slightly passably, I might add. Adding a third dimension of movement to combat requires a TREMENDOUS amount of computing, and usually doesn't work right, or well. Again, pointing the proverbial finger at Aion. Even limiting the location you can be in flight in Aion didn't fix the looming issues, namely exploitable actions, techniques, and movements monsters couldn't take. For example, flying as a ranged class while fighting a melee only monster would lead to one of two actions, either resetting the monsters AI and just making it invulnerable, or it would gain a new attack to shred your flight time until you were forced to ground once again within seconds, even if you have minutes of flight time left. As admirable a concept as it is, it has been proven by now to be a concept that sadly just doesn't work. I am sorry to be the one to burst your proverbial bubble.