Making this argument again because eventually it needs to happen.
Undead Paladins are cool. Theyre cool, alright? But just throwing them on as an available class would diminish what it means to be undead AND what it means to be a paladin. Theyd be incredibly popular, and there would have to be a new ingame subfaction similar to the Sunwalkers to justify their existence.
To add more combinations, we'd have this endless stream of watering down of the races, and the classes, to ultimately mean nothing. Which just doesnt flow right with me. Draenei hate warlocks, Tauren aren't sneaky, and goblins are not friends of nature.
Unless....
We had to unlock them. Instead of an entire subfaction, there's a side-quest in a random zone that explains the existence of ONE member of a race who has become an unlikely class. Instead of the entire racial city accepting these as an everyday occurrence, we get more characters like that Night Elf Paladin in Legion, who would then appear as class trainers for a small number of like-minded individuals.
The Draenei would still canonically hate warlocks, but a lone "fight-fire-with-fire" mage dabbles in Fel in a hidden workshop, and she's not above sharing her knowledge at the encouragement of Void Elf allies. Complete the quest, boom, unlock Draenei Warlocks.
Given the precedent of Allied Race Death Knights receiving an alternate starting experience, even Demon Hunters would be on the table for non-elf races, specifically locked behind a Demon-Hunter exclusive questline where a post-Legion Illidari trains some unlikely recruits.
Undead paladins regaining their true connection to the Light. Blood Elf botanist druids from Tempest Keep. Goblin Druids who find profit in Going Green. The possibilities are endless, without stepping on any toes or spitting in any faces.
Another thing is that it'd be a lot easier to continually add these, like one per patch. Because they don't need to create new hairstyles and such like a race, just shoehorn in an extra quest that introduces one of these strange characters.