Part of me would prefer high ground acting as a variant of cover. It'd be more interesting if (Assuming high ground Advantage/Disadvantage is removed):
  • A character could get cover for moving characters behind another allied character [Ally(+2AC)---Ally---Enemy]
  • A character could get cover for standing on an elevation at least 12 feet higher on the vertical axis [Ally---15 foot hill---Enemy(+2AC)]

Then we can have more clarity on to-hit throughout the whole game. (It could also work as a -2 to-hit).

Part of the reason I prefer cover to flanking in 5e is that it adds strategy without being overbearing. One campaign I'm in had flanking giving advantage and the group voted to remove it after 7 sessions. We all agreed it was too easy for both sides to get advantage. We had several encounters where we all had advantage.