Sounds like a pretty good idea. I know at my tables colossus slayer has always been the pick because it works (almost) all the time against any creature. Horde breaker and giant killer have higher potential since they are full attacks but aren't always applicable like colossus slayer. I feel like this new favored enemy would see rangers picking colossus slayer at level 3 most of the time and level 6 the rest of the time. It's just too good to leave until level 15 and you know you'll get one of the other two by level 6. A slight nerf to colossus slayer and I think you're in business.*
The other change I would make is keep natural explorer but also give each terrain a unique bonus. Examples: arctic (cold resist), coast (swim speed), desert (adv on initiative), forest (ignore nonmagical difficult terrain), grassland (+5 speed), mountain (climb speed), swamp (poison resist), underdark (darkvision or improved darkvision). Other rangers eventually get 3 as normal but Hunters could get an additional one or two as a subclass feature(s). There's no reason we can't also add feywild, planar, or other terrains to this list or as unique subclass terrains.
I realize this might nullify some other class features like Land's Stride and Roving. I just think if you gave these new features to rangers as well as the features from deft explorer and primal awareness you'd really be on your way to a very outdoorsy ranger. Some of these would need to be spread out more across the levels to avoid an overload in tier 1.
Ranger levels could be:
- Canny, upgraded Natural Explorer
- Fighting Style, Spellcasting, Primal Awareness
- Subclass, Favored Enemy
- ASI
- Extra Attack
- Subclass, Natural Explorer
- Favored Enemy, Defensive Tactics (from Hunter)
- ASI
- Expertise
- Subclass, Natural Explorer
- Favored Enemy, Tireless
- ASI
- Feral Senses
- Subclass
- Superior Hunter's Defense (from Hunter)
- ASI
- -
- Foe Slayer
- ASI
- Epic Boon
*I would try switching some of the benefits between colossus slayer and giant killer. Colossus slayer becomes "When a creature below it's maximum hit point total within 5 feet of you hits or misses you with an attack, you can use your reaction to attack that creature immediately after its attack, provided that you can see the creature." Giant Killer becomes "When you hit a Large or larger creature with a weapon attack, the creature takes an extra 1d8 damage. You can deal this extra damage only once per turn."