A +2 for High Ground at best, and no penalty for low ground since the enemy on high ground can sometimes choose to move out of line of sight where you can't even target them. A +2/-2 swing would still be so powerful it would be the focus in every fight. Let's not forget you can Shove too when someone tries to get to your hill. +2 and the range increase would be plenty of reason for archers to seek high ground.
Not all fights should be about "who gets the high ground". There are choke points, chasms, streams, big trees, corners, pits, trenches.......high ground is not the only terrain with tactical valule. No, I didn't say fire surfaces.. Larian... no.
Range should also be a bigger factor with ranged weapons. With high ground you can shoot really far with the same accuracy as point blank range. It's crazy seeing some of those shots with the same 94% accuracy.
I would also very much like to see climbing cost more movement. You should be able to break a ladder with someone in the middle to make them fall. No one is ever in the middle. It's a teleport with a really slow animation. And roll some Athletics or Acrobatics checks when climbing or jumping in combat to reach that high ground or wherever. What are those skills for, currently?
Backstab is a silly overpowered feature that has become the default attack with free advantage. Just not for AI, only players. Turn it into Flanking that requires at least two attackers on different sides, and grants +2 attack so that spells like Faerie Fire are still useful as well.