Positioning around height is definitely tactically interesting and its something that's hard to get working outside of the format Larian are working with so I can understand high ground being pushed. You really want the bonus to be something the player finds it difficult to ignore, especially in the early game.
I still think Advantage/Disadvantage is too much though. It wrecks the internal balancing by making anything that works on a ranged attack roll overwhelmingly better than other attacks and going from all your attacks have advantage and all attacks against you have disadvantage to the reverse is far too huge a difference. I like the idea but I think balancing it is going to be tough.