@Zellin
Advantage and Disadvantage
Good point about height actually making it harder to hit people below you, since they have smaller visible cross sections (~head and shoulders instead of full body) so that players standing up effectively look prone to your vantage point. Conversely, prone characters should be much easier to hit from height.
1.1: Maybe instead of Height giving Advantage to shooting, it
--a) extends the weapon's range and/or just doesn't give disadvantage to shooting outside the normal range
--b) doesn't give disadvantage to shooting prone characters
This solution is somewhere between what is currently implemented and your suggestions
1.2 Why do you say shooting upwards is different than downwards? Shouldn't the enemy's profile be the same either way? (In your diagram, the "difficult shot" should be equally difficult for the elevated guy shooting downwards or the ground-level guy shooting upwards, no?)
1.3 I agree with your "exception" listed here. Since "hide" is a bonus action, this suggestion would still allow backstabbing each turn, but it would actually cost some resource which would prevent also bonus action attacking, jump disengages, etc.
The other part to your suggestion-shouldn't get the backstab advantage against an enemy who attacked or was attacked by the same character in the previous round or this same round-is probably too convoluted of a mechanic. I think its best to stick with always-applicable mechanics, and not have general rules dependent on who you hit last round.
2. Possible Additions
I'm wary of adding additional sources of advantage/disadvantage. Your suggestions make logical sense, but would probably have too significant of an effect on game balance to make them worth the effort.
