The Death Knight and Demon Hunter were World of Warcraft's first two "hero classes," which is a somewhat ironic name.
In fact, in WoW's entire 22-year history, the game has only added four full classes to the original 9-class roster: the Death Knight in 2008's Wrath of the Lich King, the Monk in 2012's Mists of Pandaria, the Demon Hunter in 2016's Legion, and the Evoker in 2022's Dragonflight. In fact, of these four, only the Monk was not a "Hero Class." Hero classes required you to have a character of a certain level (usually the previous expansion's level cap, though Death Knights only started at level 55 when Wrath raised the cap to 80). The logic here was that it would be weird to have a low-level Death Knight going around in low-level areas doing relatively low-stakes quests, and so they wanted to start them off with a fair degree of power.
The game has changed significantly since 2008, of course, including the introduction of level-scaling, so you can, in fact, now level up a low-level Death Knight in regions like Westfall (traditionally the second zone human characters go to when they move on from their starting zone in Elwynn Forest).
While Evokers were basically unprecedented in the lore prior to this, the Death Knight and Demon Hunter were both the classes of iconic characters from Warcraft III, specifically Arthas Menethil and Illidan Stormrage, respectively. WCIII had a sort of vague RPG system within its real-time-strategy campaigns, where you'd have kind of hero unit that would be leading your units in all the base-building and such, and would be a formidable force. These hero units were the major characters of the story.
While the Death Knight has plenty of precedence in lots of fantasy RPGs (and I'd argue also in fantasy media in general, with figures like the Ringwraiths from Lord of the Rings and Darth Vader both occupying that archetype,) the Demon Hunter is a bit weirder.
To be clear, there certainly are Demon Hunters, or monster hunters in general in fantasy media. Indeed, one of the classes in Diablo III is also called a Demon Hunter, but I think that it adheres much closer to the way that such a character class is usually portrayed: in Diablo III, the class is more like Diablo II's Rogue (and I guess also Diablo IV's,) with a flavor of having lots of cool gadgets and traps to fight demons, but basically being your black-clad, acrobatic, kind of Solomon Kane-like figure.
WoW's Demon Hunters have an entirely different aesthetic, strongly informed both by WoW's Night Elf culture and also the aesthetic of its demons.
In WoW, a Demon Hunter is not just someone who hunts demons. Indeed, there's actually a Hunter class, which is much closer to D&D's Ranger, typically fighting with ranged weapons and/or an animal companion, with a lot of "nature survival"-themed abilities along with their weapon attacks.
The premise of the Demon Hunter is that it's a highly specialized path that fundamentally changes you on a physiological level. Demon Hunters don't just use fel magic - they literally absorb a demonic soul into their own, becoming half-demons themselves. Demon Hunter characters typically have horns, and might have scaly skin or cracks where the bright-green Fel energy shines through.
Part of the ritual to become a Demon Hunter involves gouging out one's own eyes. The fel energy inside replaces them, allowing Demon Hunters to see, and even see through disguises and invisibility. Thus, as a cosmetic option, Demon Hunter characters can select a number of blindfolds, or they can just let the bright glow of their fel-filled eye sockets shine unencumbered.
While the current expansion, Midnight, introduced a third specialization (WoW's version of a subclass) that both allows them to act as a ranged spellcaster and channel Void, instead of Fel magic (which is color-coded in blues and purples, as opposed to Fel's neon green and occasionally deep red,) for the most part, Demon Hunters fight in melee, dual-wielding weapons - most iconically, Warglaives, which are massive, double-bladed weapons. They are absolutely fantasy BS kinds of weapons, because A: they would weigh a ton each, and B, it would be hard not to slice your own thighs while swinging two of them around at your foes. But in a video game, they look really cool.
Anyway, the motive of Demon Hunters is to use the power of demons to fight demons - in WoW, basically all demons are part of a massive interstellar army called the Burning Legion that seeks to conquer and destroy the universe - the conflict that started the series was caused by the Burning Legion manipulating the orcs into acting as a proxy army to invade Azeroth, which they had failed to conquer 10,000 years earlier thanks in part to people like Illidan Stormrage, who basically invented Demon Hunters and was the first one.
I do think there's some logic to building Demon Hunters on the bones of the Ranger, conceptually: while very different in aesthetic from your typical Ranger (again, WoW's Hunter class is the clear equivalent,) the Demon Hunter does have a focus on taking down monsters (even if in WoW, they've obviously got to be fine at fighting any type of foe. We haven't really had demons as a major threat since Legion, which ran from 2016-2018). Ironically, this might have made them a better fit for the 5.0 Ranger, with its Favored Enemy mechanic (naturally picking "Fiend" as your first favored enemy type).
As we talked about with the Death Knight being represented by the Oath of the Ebon Blade subclass for Paladins, I suspect that Demon Hunter Rangers will also see some significant changes to the level 1 and 2 class features, as hinted at in that James Wyatt interview.
Oddly, I almost think you'd need more distinct features than the Death Knight - DKs in WoW do have some cosmetic differences, such as glowing blue eyes (at least starting with Wrath of the Lich King, the power of necromancy is often represented with a bright, icy blue, though this was more specifically the color associated with the Scourge and the Lich King, as there are several colors of death magic when we went to the realm of the dead in 2020's Shadowlands expansion) and a few partially-rotten skin textures you can pick, but it's not nearly as dramatic as the Demon Hunter customizations (which was maybe just because of 8 years of development).
There are various aesthetic things I'd really suggest should be part of the Demon Hunter subclass, and represented mechanically:
First, the missing eyes. It might be too powerful to give them a baseline 30-foot blindsight, but perhaps they can activate blindsight in a reasonable range a certain number of times per day. This could upgrade to Truesight at higher levels.
Next, their tattoos: the Demon Hunter aesthetic is classically one with a lot of skimpy clothing: Illidan would typically go shirtless. In WoW, your armor is a huge part of your character progression, but when designing Demon Hunter-specific sets, they've often made them more revealing. Demon Hunters have magical tattoos that help protect them, and thus, I think giving the subclass Unarmored Defense would make a lot of sense - Dex+Wis would work fine, just like it does for a Monk.
Then, we've got their wings. In WoW, Demon Hunters can double-jump and also pop out wings to glide down and prevent falling damage. Maybe this is a Slow Fall-like feature, or maybe we set this one aside.
Then, we have Warglaives. In WoW, this weapon type is available only to Demon Hunters (and thus is always statted-out to be ideal for them, with the addition of the spellcasting Devourer spec adding Intellect to all Warglaives in addition to the usual Agility). There's not really an obvious equivalent to them in D&D. I might be tempted to make them d8 Light weapons, though perhaps for balance they need to be d6s. The other issue is that, thanks to Weapon Mastery, we'll want to either have two types of Warglaives (one with Nick and one with something like Vex or Sap,) or Demon Hunters will, weirdly, find themselves probably not dual-wielding them, which feels wrong. (Fun fact, in the expansion that Demon Hunters were made playable in, every spec got a single weapon or set of weapons for the entire expansion that simply got upgraded over the course of it, so Demon Hunters naturally only used Warglaives the whole time. It was only later that they started to use the axes, swords, and fist weapons that they were also capable of using the whole time). (Also, ironically, when we fought Illidan in 2007's Burning Crusade expansion, he had a small chance to drop his legendary Warglaives of Azzinoth, but because that weapon type didn't exist yet and the class that could use them wouldn't be there for 9 more years, those actually just count as one-handed swords. Illidan kind of got handed the villain ball in BC, but Legion did a lot to rehabilitate him as the rash but ultimately well-intentioned anti-hero he was always supposed to be).
Now, how much of this is going to be stuff we get at level 3? No clue. I actually have far less of a sense of what Demon Hunter features will be subclass features and what will be spells we can pick up. Even more than the Oath spells for the Ebon Blade Paladin, I feel like Demon Hunters have to get more than the typical "one per spell level" subclass spells that Rangers typically get. We know Demon Spikes will be one of them (a defensive ability used by Vengeance Demon Hunters, the tank spec for the class).
Still, I think that mobility has to be a major theme - Demon Hunters have a ton of mobility features in WoW, and a big part of their overall vibe is being "speed demons," so to speak (one of the major rotational AoE abilities for Havoc, their melee damage spec, called Blade Dance, has them rushing faster than you can see in a kind of X pattern, slashing all foes in the surrounding area.)
I will say, I'm far less enamored of the Ranger class in D&D than the Paladin one, so I'm a little more concerned over whether the Demon Hunter will be appealing to play in an Azeroth-set game. But we shall see.