The WoW D&D book is going to give a lot of guidance for player to re-create WoW classes and specializations within D&D rules. In some cases, this will be trivial: a Guardian or Feral Druid can just go Circle of the Moon (though I'd freaking love it if they create a scaling template for Bear and Cat forms we could use at any level). But in some cases, namely the Death Knight and Demon Hunter, they'll be doing some unprecedented re-works of existing classes and creating subclasses that are sort of more than subclasses and really extensive re-works of the base classes they're being grafted onto (we've only seen level 1 and 2 features for Ebon Blade Paladins, which replace Lay on Hands and Paladin's Smite - though Rune Strike, which replaced Divine Smite, is basically the same spell except it does Necrotic damage and only gets a bonus die against Celestials).
But while it could be fun to play with restrictions to require players to build characters to fit with WoW's existing classes and specs, I think think it would be more fun to take adventures in Azeroth to places you can't replicate in the MMO.
Let's consider the Warlock.
In D&D, of course, a Warlock is a character who has forged a pact with a powerful magical entity (though 5.5 broadens this a bit to allow you to draw from multiple sources, as long as they're sort of in the same category.
By contrast, Warlocks in World of Warcraft are scholars just like Mages (the Wizard equivalent,) but while Mages study in very socially-acceptable learning institutions (though for a long time, their practices were forbidden among the Night Elves,) Warlocks are scholars of forbidden knowledge, specifically the knowledge of Fel magic, which is the inherently destructive power that suffuses demons.
Warlocks in WoW have three different specializations, like most classes, but all three are still Fel/demonic magic. Affliction is the one that might be easiest to reflavor as instead some kind of necromancy, but the other two, Destruction and Demonology, are both firmly in that Fel/Demon category (Destruction focuses more on blasting foes with demonic fire and raw chaos while Demonologists summon tons of demons to attack their foes).
D&D's Warlocks, thus, have a broader array of options in where they get their magic, and so I figured what I'd do is go through each Warlock subclass and talk about what kinds of patrons you might choose.
Archfey:
While WoW doesn't have a Fey creature type, they do have some entities that would probably fit with the Fae, most notably the explicitly named Night Fae, who are the caretakers of the realm of Ardenweald, one of the afterlives in the Shadowlands. Here, natural creatures and people dedicated to nature go to take part in the cycle of life, eventually being reborn in a cycle of reincarnation. The Winter Queen is the leader of the Night Fae. But more broadly, the Wild Gods might fit this category - Wild Gods cover a lot of animal spirits worshipped or revered by various cultures across Azeroth. The Night Elves have their Ancients (not to be confused with the giant walking trees also called Ancients, who are... also nature spirits of course, but in a less divine way,) the Trolls have their Loa (which also include some beings that aren't quite as animalistic or even associated with nature) and the Pandaren have their August Celestials. Even the Orcs have animal gods like Lo'gosh (who might actually be the same as the Night Elves' Goldrinn...?) While not necessarily as fickle and tricksy as Fey tend to be, these are probably your best equivalents.
Celestial:
It's really up in the air which of WoW's creatures would count as Celestials, but the ones that I don't think you could argue against are the Naaru. These are crystalline beings of pure holy light that are particularly important to Draenei culture. While it's supposed to be very rare, in game we often see Naaru go through a horrible cyclical transformation, turning to the Void as Dark Naaru (who are probably more on the Aberration/Great Old One side of things). Other options potentially include, actually, the Titans or their Titanforged creations. While the Titanforged act very much like constructs, beings like Tyr, Odyn, Thorim, Freya, Mimiron, or Ra-Den (the latter of which is dead by the end of Battle for Azeroth, but then, Tyr was dead for a very long time before we brought him back during Dragonflight) I think they'd fit pretty well in this category as well.
Fiend:
I mean, there's plenty of demons in WoW. Warlocks in World of Warcraft tend to invert the patron relationship, with demonic minions bound to serve Warlocks through contracts that make any Warlock worth their salt the more powerful person in the relationship, but bargaining with power from a powerful demon certainly feels plausible on Azeroth. I think you could play a Fiend Warlock easily enough as a standard WoW warlock (just remember that Fel fire is typically green!) Another possibility is actually Illidan Stormrage. While sort of a half-demon (though he sure looks more demonic than even the Demon Hunters who he's trained) Illidan is WoW's ultimate edgelord antihero, a firm and hard Chaotic Neutral. (Also, Critical Role fans, if you didn't know already for some reason, will be excited to know that he's voiced by Liam O'Brien!) But also a somewhat more benign source of Fel power.
Great Old One:
Two really great categories here: The most obvious is the Old Gods. While all four of Azeroth's Old Gods are (in theory) dead in the current state of the game, we did have to kill three of them as the final bosses of their respective raids (well, there was an optional extra-hard boss in Ulduar after Yogg-Saron). But these ancient flesh-mountain monstrosities certainly fit the bill. Alternatively, you could go for something less fleshy and more cosmic with the Void Lords. The only one of these we've ever actually seen or had named is Dimensius the All-Devouring (who was the final boss of the last expansion) but I think you could lean into the mystery of what your patron is if you went with an unnamed Void Lord. (The game's current antagonist, Xal'atath, and likely the final boss of the current expansion, is a former mortal turned void-entity that was probably not a true Void Lord, but might fit this, though she'd be an absolute perfect Hexblade patron).
Undying:
This subclass... is weird because it could just be an undead patron, but it's also explicitly not just undead beings. If you for some reason really wanted to play this not-very-good and conceptually thin subclass, though, there are tons of extremely old beings on Azeroth. Maybe you go with Nozdormu, the Bronze Aspect, leader of the dragons who manage and safeguard the timeline (or his alternate-timeline-self nemesis, Murozond).
Hexblade:
There are a few sentient weapons, most notable of which was Xal'atath, Blade of the Black Empire, which was the artifact weapon granted to Shadow Priests during the Legion expansion, but whose resident conscious being transferred into the body of a dead elf (that we provided for her) and has now been leading a deadly assault on Azeroth with her void-creature army called the Devouring Host. Two other weapons for Legion had personalities, with Aluneth, a staff that contained a powerful arcane consciousness, and the Skull of the Man'ari, the remains of an Eredar summoner who got into demonology before it was cool on Argus and ironically was arrested by future demon lord Archimonde for dabbling in such magic. Other weapons like Ashbringer, Doomhammer, or Frostmourne might fit here, though it's dubious whether they're actually sentient (technically I guess Frostmourne might be a conduit for the Jailer).
Genie:
WoW does have genies, but they're a pretty rare category specifically of air elementals. If you wanted to take the idea of a powerful elemental, you could possibly instead go with the Elemental Lords, of which there are conveniently one for each element. The OGs are Ragnaros the Firelord, Al-Akir the Windlord, Neptulon the Tide-Hunter, and Therazane the Stonemother. However, there's been some turnover over the course of WoW. Ragnaros is dead, and was replaced by Smolderon, though we also had to take him down as well (though this was outside of the Firelands, so I actually think he's probably still alive and still the Lord of Fire). We killed Al-Akir as well, though his son Thunderaan I believe remains the Windlord. Basically, depending on when in WoW's timeline you want to set your campaign, you might need to pick alternatives. Note that the elemental lord of Draenor, the Orcish homeland (and sort of adopted Draenei homeland, though they were only there for about a thousand years, which isn't much for a people who seem to naturally live to be tens of thousands of years old) you could pick them.
Fathomless:
Ok, there are probably some other options here, but I think the best one is Queen Azshara. Once the radiant and beautiful queen of the Night Elves in ancient times, when her capital sank beneath the waves, she struck a deal with the Old God N'zoth to save her people by transforming them into the serpentine Naga. Azshara is one of Azeroth's most dangerous villains, but she's a pragmatic and patient one (while her royal palace in her capital, now called Nazjatar, was a raid with her as the final boss, she survived this attack and we actually wound up rescuing her from N'zoth, a decision that will probably bite us in the ass somewhere down the line).
Undead:
Boy howdy. So, the obvious options here would be members of the Scourge. The most powerful of which is certainly the Lich King. But if you're playing in a post-Wrath timeframe, the Lich King could be Bolvar Fordragon, a heroic paladin who sacrificed his freedom to try to hold the Scourge on a tight leash, and while Bolvar certainly did some questionable things as Lich King, he was certainly more benevolent than Arthas had been. WoW does have its fair share of Liches and Death Knights. I actually think making the entire Knights of the Ebon Blade a patron could be fun, or perhaps specifically having the Four Horsemen, the Ebon Blade's most elite knights (aside from the Deathlord, the player character, of course) as your patron would allow a DM to play the varying personalities between them (this latter option doesn't totally work if you're doing the Scarlet Monastery dungeon, as I'm confident its final boss will be High Inquisitor Whitemane, who is later raised as a death knight and becomes one of the Four Horsemen, ironically "redeeming" herself by becoming an undead killing machine. Better than a religious fundamentalist!)
Vestige:
This new subclass from Arcana Unleashed ironically might actually make you play a little more like a WoW Warlock, who generally have a minion fighting alongside them in battle. (I play a Demonology one, who would always have a minion out, but I can't remember if the other two specs even have the option to not play with one). But mechanics aside, I do think there are some options here, even if they're a bit non-obvious. We have a few dead god-like beings, such as the nascent Titan Argus, or the lost (and very much beloved - the fans got up in arms) bear ancient Ursoc. Rezan, the Zandalari Troll Loa of Kings (he's a T-rex!) died to the treachery of the Prophet Zul. It also turns out that the Titans were, actually, for most of WoW's history, already in a state of more or less death - Sargeras slew them right around the time that he founded the Burning Legion, and it wasn't until we liberated their tortured souls at the end of the Legion expansion that they were able to return to their full power.
I think that's all of them? I'm actually quite pleased that there seems to be at least one pretty good option for every subclass, and there are probably more if you go digging.
Ironically, though, I think you could argue that these subclasses almost see representation in other classes in WoW. Shadow Priests, for example, have some abilities based around the Old Gods. And actually, the lore of the Shaman class is that they act essentially a mediators and negotiators between elemental spirits, which I actually think is pretty Warlock-coded (though it seems they'll be represented more by Barbarians - which is surprising on a mechanical level but sort of makes sense as two classes really deeply rooted in ancient/primal powers).