In fairness, yes, there is at least one example of an Undead Paladin so Undead Paladins can be a thing.
https://wow.gamepedia.com/Sir_Zeliek
https://wow.gamepedia.com/Leonid_Barthalomew
I am unsure if Leonid is still a Paladin or if he merely has faith in the light as a leftover from his days alive.
While Undead Paladins do exist, their rarity and the fact players don't have the option to select them mean they are intended to stand out when they do occur, to be a freak occurrence due to the depth of the faith of that particular Undead in the light. It is important to note that almost every single Undead was raised to this status
unwillingly, no more how light fearing they were. To retain their powers in the light, a light that has abandoned other unfortunates who were unwillingly corrupted (think of Nobundo losing his Paladin abilities when corrupted by the fel), means that these individuals possessed an almost unparalleled faith.
Void Elves in contrast never had that faith to begin with. Either they were researching the void with Umbric when transformed by the Nether Prince, or they sought out the Void Elves to 'learn to wield their powers' as Moorgard put it. No follower of the light would deign to learn to use the shadows.
Priests are not a good counter-argument, as it is quite clear that some races do not mesh with either the light or the shadow side of the Priest. Lightforged Draenei would be extremely unlikely to use the powers of the shadow, whereas Void Elves almost by definition would be extremely unlikely to use the light. Yet everyone accepts that those races need to be allowed to use those specs for game mechanics purposes, and this is a classic example of gameplay trumping lore.
Void Elf Paladins are not like Undead Paladins. The few Undead Paladins set themselves against the grain and exist primarily because they were Paladins while alive. Void Elf Paladins are an oxymoron, a race-class combination that contradicts the core theme of the race itself.
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I believe Blizzard counted Alleria's story as the Void Elf story, as her arc in 7.3 was the tale of how the first Void Elf came to be. And Alleria most definitely had a role in Legion.
BFA is more about the Old Gods, who serve the void and use shadow magic. I presumed there would be less of a distinction than there has turned out to be, but in time we will confront the Void Lords and at that point, the Void Elves will have a role to play in the story.
Part of me hopes that is a misunderstanding on the part of the interviewer, which has been known to happen, as the last thing the game needs is a new warfront.
And yes, the storyline can be told through another mechanism other than warfronts, but that again presumes there is a story to tell. There seems to be a certain elf-centric chauvinism that only their story deserves attention and needs to be told. There seems to be a very heavy elf focus in 8.2.
8.3 has to wrap up both the current N'zoth storyline, the end of the faction war and Sylvanas's replacement as Warchief. If Quel'thalas is to feature anywhere, it would be in the end of the faction war storyline, but if such a thing were to happen it would be about the end of the faction war and not fixing the problems you seem to have with Void Elves.
Because this isn't foreshadowing. The Void Elves have a connection to the shadow and the void. So do the Old Gods.