I agree with OP wholeheartedly, but I think a straight +2 to hit for highground could be potentially too much when combined with advantage from another source.
Here's my slightly modified solution: -You have high ground bonus(+2) and your target has been Faerie Fired, granting you advantage. -You make your attack and roll your first d20 with a +2 for high ground -You make a second roll, because you have advantage, but you don't gain the +2 for high ground -You then take the highest roll
This is admittedly a pretty strange solution, but I think it would help with hit chance bonuses getting out of hand. It allows you to gain benefits from both sources, without having them synergise into an auto hit.