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.