You'd be pretty much treading the same ground as deathknights by then, even more so than it would already do from the onset. Unless you want the dark ranger part to be only wearing cloth or the caster to wear only leather or higher - both of which would be a betrayal of the underlying fantasy of the specs - you'd essentially just end up with another weird hybrid that is lacking the full fantasy of either. Which is pretty much the only argument why unholy DKs don't fullfill the necromancer niche sufficiently, because besides the armor and range they already heavily deal in dots and minion gameplay.
Incorporating some dark arts into the hunter class wouldn't be much of an issue as well. There is really no hard lore that says hunters need to be pro life, heck we have mechanical pets already and dark rangers aren't required to be undead either, since the living can channel necormantic magic as well. Not to mentioned the half rotten beasts that frosaken (and other) hunters can tame. In essence dark rangers are just fallen rangers (so the good parts of hunters) anyway. Heck you could even make a little quest where the dark ranger from the legion order hall is looking for sylvanas in the maw or some shit and you unlock it that way; it practically writes itself. I see no reason why you'd need to make a whole class about it whose only commong ground is some use of necromantic magic. Dark Rangers are not necromancers and necromancers are usually not trained in archery.
Necromancer on the other hand has quite a few more issues to make it into the game. While a similar logic to unlocking it could be applied to warlocks and worst case even mages, warlocks and necromancer have really nothing in common other than dealing with life forces and souls occasionally. Warlocks have nothing to do with the dead though, and necromancers usually don't deal in demons either. It's always weird when combining classes and suddenly core features no longer work in certain specs, which is definitely the case with the 4 warlock summons. But the bigger issue rather than the core fantasy is the origin of WoW's warlock class in itself. Warlocks are the "more acceptable alternative" to necromancers in WoW's lore because necromancers were wholsale connected to the scourge. Warlocks are just power hungry, necromancers are flat out the enemy of life. That is also why warlocks have the classicial gameplay tropes of debuffer/DoT gameplay as well as minion summoning in WoW associated with them, when other franchises don't use them like that. In the RTS games they were also the "evil orc" stand in for mages, which mostly explains the destruction tree. In WoW necromancers are practically already existant as warlocks when it comes to gameplay, at least if we talk classical necromancer gameplay. A different take on necromancer like in ESO may be an option, but that is really not what people are usually asking for when the clamor for a necromancer class.
While I think after Drakthyr everything is possible given enough desperation on Blizzard's end, it also stands to reason that the time for another necromantic class has simply passed for now. I'd argue that we won't be seeing them any time soon, until we retread the undead theme again, which is probably off another 5-8 years at least.