If we assume that Larian will be keeping the high ground / backstab advantage system, a few minor changes will help solve a lot of problems.

1) Implement Backstab as proper Flanking

A simple way to ensure backstab bonuses don't make other advantage sources obsolete is to implemented it like the optional flanking rule in the Dungeon's Master Guide. Essentially, the game assumes that in 1 on 1 scenarios, your opponent will always auto-face you (denying you the backstab advantage). A second person, the flanker, is always required to actually backstab.

This simple change will ensure special advantage sources - like Reckless Attack, will still have a purpose. It'll also limit the brutal amount of jump -> repositioning that currently happens in a melee fight. However, repositioning and proper teamwork will allow you to reduce RNG.

2) Implement a better cover system to counter height advantage

If Larian is going to keep the height advantage to hit mechanic, then they should implement a proper cover system from the rules to counter it. The DMG values are:
  • 1/2 Cover = +2 AC
  • 3/4 Cover = +5 AC
  • Full cover = can't be targeted

A proper cover system (where it shows you if a ranged attacker's weapon LOS- i.e. like Wasteland 3), will allow both the player and AI to make tactical decisions/tools to actually deal with high ground advantage.

Small changes to the AI - i.e. maybe enemies don't just use fire-surface-area arrows on you as soon as they can, but reserve it to flush you out of high ground / cover, would add more tactical depth to this engagement.