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This is a great idea, but I wonder, why not go further?
Favored Enemy : Whoever I just tracked and watched from afar, or looked at their past kills, or researched their nests or hunting grounds for patterns. Think about Aragorn seeing the tracks of the hobbits escaping during the scuffle of the orcs + uruk'hai. He can imagine things, recreate them in his mind's eye...like a sherlock holmes of battle tactics against monsters or monstrous humanoids. I'd rather that was their niche, than just "humanoids", which IMO would impinge too much on the Fighter's niche. A fighter should be good against anything...but Rangers excel at killing...and not being killed by...big game monsters out in the wilds.
How the rules achieve this remains to be seen. But while this idea is VERY close to the ideal thing I'd like to see, I'd rather they just ignore entirely the racial component, and focus on a tactic to be countered. When you encounter a new monster tactic, you can learn it to apply disadvantage against their attacks (possibly even during the same combat, if you roll well), and advantage to attack them.
Rangers should excel at finding the one place in the dragon's belly where there's a missing scale, or anticipating when it's about to unleash their next breath weapon. I'd rather this be a dynamic, exciting, roll ...based on a class feature...such that as you travel, you gain benefits against enemies with which you've successfully defended yourself or have killed, such that the next time you face them, you have the upper hand. (or less of an insurmountable challenge).
I like "learn by doing" skill / combat systems, similar to EQ. In D&D, a fighter used to gain XP by killing monsters. Rangers should gain XP by killing them in one hit. To me, this means learning to crit them, or learn ways to gain advantage against them. That means, giving a way for each enemy in the monster manual a way to both grant advantage, or conditions that would cause its own attacks (or some subset thereof) to be disadvantaged, against the ranger.
A ranger's defense isn't his AC, it's finding ways to apply disadvantage against the BBEG's big hits. His offense is similarly, finding ways, through experience, training, perception, and clever deduction, of gaining himself advantage for his own attacks. He may, even, tell his buddies of his discoveries as he goes along.
Win