Attacking someone from the rear is already in the rules. If you attack someone who cants see you; you get advantage. Its not taken from Divinity at all.
This is the optional
facing rule the other guy just posted here. But even that rule allows following: "
A creature can also change its facing as a reaction when any other creature move.". You are not supposed to just walk or jump around somebody's back and suddenly get the strongest buff in the game.
I agree something like ±1d4 or flat ±2 for highground should be fine. That way spells which gives you advantage still hold its value. E.g you can combine high ground with Guiding Bolt. Or True Strike, while bad spell, still would have at least some sense.
