Nightborne are Highborne thing, not night elf or Blood elf.
The introduction of the nightborne has obviously caused quite a buzz about the elven community... lots of talk about the blood elves getting a new addition, a handful of people feeling it's a night elven addition, a lot also thinking it's a completely new group/thing set up to be a 3rd elven main race.
Whiles we cannot deny there is a similarity to the blood elves who share a lot in common with the nightborne, and while we cannot deny that pretty much everything about the nightborne is night elven in origin very few have realized the overwhelming HIGHBORNE connection that speaks louder than every other vein. The night elf highborne.
Looking closely at the nightborne - it is without a doubt the arcane night elf group the Highborne that has been expanded. Whiles you could argue about the technicality of highborne being night elf, or being high elf since high elves came from that... the very high arcane night elven nature of the nightborne has HIGHBORNE screaming out very loudly emphatically the the nocturnal group, not the diurnal group - making it primarily night elf concerned rather than a high/blood elf thing (and that's not invalidating their is connection). Highborne never refer to themselves as night elves even though they have the night elf appearance, and nightborne is a lot closer to highborne than it is high elf/night elf - to me this all indicates that they are development of the highborne into a serious thing in the night elf category. It's own separate group.
There is a reason you've never felt the nightborne don't quite fit the night elf thing, that's because they're based on the Highborne group, the arcane night elf group you've really onlly had glimpses of in ruins with the Shen'dralar and not had a proper look till now. There is no longer any doubt in my mind, this is a highborne thing. And being highborne while a night elf thing it provides a bridge and strong connection to the high/blood elves.
Even the music of the zone/themes are shared with Azsuna strongly -could we dare to conclude, that blizzard is making in the nightborne the highborne into a race of it's own?
It seems clear to me that the nightborne are blizzard greatly expanding primarily the Highborne, and secondarily the night elf