Advantage and disadvantage.1. Already existing ones.1.1. High ground advantage with ranged weapons should get it's proper relation to ranges.
It's not even partially easier to shoot downwards from most of the medieval ranged weapons. That's why shooting prone characters is disadvantageous. For a character, that stands atop of something, the characters right under are pretty much the same as prone.
So here is the scheme of how high ground should affect shooting more realistically.1.2. High ground shouldn't cause disadvantage on attacks against the character on the high ground, when the ground doesn't work as cover.
Unlike shooting downwards from a medieval weapon, shooting upwards is not a problem. And with the melee weapon, if you can reach your enemy with it, there is no additional problem at all, you can just aim for legs. There is even the Mythbusters episode on the matter.
1.3. Backstab advantage should consider possible enemy awareness about our position.
While it's true that stabbing someone from the back is way easier, it's not true that someone would easily allow us to backstab him in the middle of a fight, if he saw where we were going. Most likely that person will jump away from the possible backstab right after seeing us moving towards his rear. So we 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. The exception can be made if we successfully go in stealth between those attacks.
2. Possible additions.2.1. Would be cool to see cover mechanics fully implemented.
I believe it's possible to make it right by the book.
2.2. Add disadvantage with melee weapons against characters that are standing in fire.
Simple logic, it's hot there and moving close towards fire should cause considerable discomfort. And it can mitigate fire being the answer to everything.
2.3. Add disadvantage on attacks from characters, whose movement and precision are somehow hampered by elemental effects: burning, being on ice e.t.c.
