I was just thinking that player ‘dark ranger’ (as in undead with pale skin and red eyes) customizations could come at a later stage for both Blood and Night elves, when completing a specific questline, just as it happened when the night warrior customization was added for the night elves.
The theme would totally fit Shadowlands, and it could easily be a patch feature: easy to make and a guaranteed seller. 
Night Elf - male and female
Blood Elf - male and female
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But dark rangers weren’t born that way!
Indeed, but that did not stop Blizzard from giving us the ‘Night Warrior’ customization as an extra perk upon completion of the storyline explaining how the Night Elves could get empowered by Elune. So all we’d need would be a quest line explaining how a number of undead elves came to rejoin their people following Sylvanas’ betrayal of the Horde.Only Hunters plus at most Rogues should have access to Dark Ranger customization.
If that’s what Blizzard wants, the customization can be limited. Death Knights and Demon Hunters show us that class-specific features are technically feasible. But bear in mind that at this time, ‘Night Warrior’ customizations are available to all Night Elves, mages included. So ‘Dark Ranger’ could simply be considered as synonym for ‘undead elf’.Dark Rangers should rather be a Hero class.
They could be, but one might argue that after Legion and the Demon Hunters, having yet another elf-only Hero class might result unsatisfying to many players not particularly keen on elves. Plus, it would feel a bit weird to have to wait for the end of Shadowlands and the next expansion for such a class considering Death is the current main theme.Dark Rangers should be an Allied Race.
Had they been released in BfA, it might have happened. But as allied races with only very limited customization resulted unsatisfying to many players, Blizzard appears to be steering away from that model and be rather keen on adding customization to the base races now, that being a core Shadowlands feature. Also, an allied race only limited to one or two classes sounds rather unlikely to be worthy of its development time.But weren’t Dark Rangers specifically Blood Elves females?
Originally, indeed. But this was already retconned in BfA... as shown by the images above.As a matter of facts, in game there is already a toy which can change players of both factions to Dark Rangers of either gender:
