The problem of +(anything) due to high ground &/or backstab is the meta gaming. When you get Great Weapon Mastery & Sharpshooter, they sacrifice their accuracy as a trade off for raw damage. If you shoot every single time with advantage means that instead of a trade off you’ll gain reliable damage. Meaning there’s no sacrifice in actions at all. You’ll never bet against the system. Also, if they implement elven accuracy, oh boy...
There are also classes that directly benefits from the advantage like Barbarians, Samurais, crowd control based classes (bards, Wizards, etc)
With that homebrew rule the game is forcing you to follow that path otherwise you gonna fail miserably. Remember that the foes will also take advantage of that. I’m seeing the 4 finesse battlemasters the easiest party composition & bards and Barbarians dropping from the A tier to D tier.
What would be “reasonable” is +raw damage instead of 2d20. That keeps the tactical positioning in the meta without causing a chain reaction in DnD5e core rules.
