Reading some of the replies has me thinking a lot of them are conflating having *an* advantage to having Advantage per the 5e rules. High ground might grant an advantage, but it doesn't make it twice as easy to hit the target. Nor does it make you twice as difficult for your enemy to hit.
The backstab rules...am I reading this right? You can just walk up to someone in the middle of a fight, slide around behind them and gain Advantage on your attack? Like he's gonna just stare straight ahead while an armed enemy walks around him...really? Plus backstab damage if you hit? Holy hellbound hobbits, that's crazy! It's so beyond the pale I'd never even tried it in any of my playthroughs.
Dear Larian: Backstab doesn't grant you Advantage. You can only Backstab if you already *have* advantage.
I like the solution of a general +2 for ranged attacks from superior height. It gives you a reason to try for the high ground, but it doesn't doom you if the bad guys get it instead.
It's bizarre to me that Larian hasn't already implemented the Flanking rules. It'd be trivially easy to do. Can you draw a line from center-mass of Character A to the same spot on Character B without touching Target C? If so, he's not flanked. If he's flanked, both A and B get Advantage, but C does NOT get Disadvantage just from being flanked. It makes combat into an ever-shifting, engaging swirl where each side tries to flank or prevent themselves from being flanked. Do you put Gale within that bugbear's reach? If you do, Lae'zel gets Advantage and will probably kill 'ol Buggie..but if she doesn't, Gale's got problems. Especially if the AI has a goblin sprint over just to flank Gale in order for the bugbear to have Advantage on *his* turn.
As mentioned by the OP a high-level fighter and a n00b aren't that far apart hit-percentage wise. Where the 17th level fighter pulls away is in having a ton more ways to gain Advantage or impose Disadvantage on his enemies.
Larian can fairly easily make fighting much more engaging by following the OP's suggestions and by implementing some simple flanking rules.