And Demon Hunters are just Warlocks with Metamorphosis. Death Knights are just Warlocks with Death Coil.
Silence (currently a Priest ability, not a Hunter one), Black Arrow (removed from the Hunter), Life Drain (Warlocks have Drain Life - like Sylvanas has Life Drain and Gul'dan has Drain Life in HoTS. sounds similar, operates similar, and yet, were still distinguished), Charm, Banshee's Curse (Saying it is unique to Sylvanas is like saying Gorefiend's Grasp is unique to Gorefiend, Illidan's Grasp is unique to Illidan, and Eye of Leotheras is unique to Leotheras. all of them were given to the Death Knight and Demon Hunter classes), Withering Fire, Shadow Dagger, Haunting Wave, Wailing Arrow, Mind Control (currently a Priest ability, not a Hunter one), Might of the Banshee Queen (see above: about uniqueness), Unfurling Shadows, Overwhelming Affliction, Unstable Poison, Mercenary Queen (see above: about uniqueness), Possession, Barbed Shot (Beast Mastery Hunter ability), Lost Soul, Festering Wounds (close in name to the Death Knight ability Festering Wound), Cold Embrace (a Warlock's Potency Conduit), Windrunner (see above: about uniqueness), Remorseless, Evasive Fire, Deafening Blast, Dark Lady's Call (see above: about uniqueness), Withering Barrage.
So, the only abilities representing the Dark Ranger within the Hunter class would be the now removed Black Arrow, and the Beastmastery Hunter ability Barbed Shot (when a Dark Ranger would be more of a Marksmanship Hunter).
A Dark Ranger Specialization, alongside a Priestess of the Moon and Sea Witch specializations, within a class. not a class on its own.
And, if there's still not enough meat on the bones for you, as a specialization, they can draw more inspiration from Diablo 3's Demon Hunter class (Valla), as they are both feminine, hooded figures with red eyes, that use ranged weapons and dark powers:
Multi-Shot (Hunter ability), Caltrops (removed Hunter Talent), Death Siphon (removed Death Knight Talent), Strafe (unavailable to players), Punishment (unavailable to players), Death Dealer (removed Death Knight ability), Gloom (unavailable to players), Frost Shot (unavailable to players), Rancor (unavailable to players), Impale (removed warrior talent), Rapid Fire (Hunter ability), Bola Shot (Removed hunter ability), Preparation (removed rogue ability), Shadow Power (removed shadow priest specialization bonus), Fan of Knives (rogue ability), Marked for Death (Rogue talent, hunter artifact trait, removed hunter talent), Thrill of the Hunt (hunter talent), Steady Aim (hunter azerite trait), Cull the Weak (removed demon hunter talent), Entangling Shot (unavailable to players), Chakram (close to the Hunter ability Chakrams), Companion (similar to the hunter's 'murder of crows' talent), Grenades (very close in appearance to a hunter's 'wildfire bomb'), Smoke Screen (very close to a Rogue's 'smoke bomb'), Vault (very close to a Demon Hunter's vengeful retreat), Spike Trap (unavailable to players), elemental arrow (unavailable to players), sentry (unavailable to players), Vengeance (unavailable to players), night stalker (very close in name to the rogue talent nightstalker), brooding (unavailable to players), rain of vengeance (similar to a hunter's 'volley' ability), cluster arrow (similar to a hunter's 'explosive shot' ability).
Personally, i wouldn't want them to take Thrill of the Hunt, explosive shot or Chakram from the Hunter, Fan of Knives from the Warden, Smoke Bomb or Nightstalker from the Rogue, nor would i want the Dark Ranger to use Grenades and Turrets. i just wrote what the class has.