Not really sure how to address this game wise vs real life wise. But shooting down at a target is much easier than shooting up at one. When looking down at something you get a much wider sense of picture at your target and immediate surroundings. Being below something and looking up at it, you only get to see your target and what's directly infront of it. I'm thinking from an elevated rampart point of view.
In game, I would like to see maybe not such a dramatic increase of damage and accuracy from having the high ground. But the exact opposite for the character that has the low ground.
The low ground character should suffer accuracy penalties. To incentive a pushed assualt for the high ground or clever thinking to neutralize the low ground debuff by finding even level terrain and repositioning.
If you’re shooting up at someone chances are they’re going to get some degree of cover, so yeah you should be penalised for that using the normal cover rules (they get +2/+5 to AC).
