IDK how NIghtborne would work as a playable race unless they were like a Hero Race, due to how their society worked until we're level 100.
But Nightfallen could work, and are unique enough in model and culture that you can't trod out "well it's just a reskinned X."
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But a Nightborne in Teldrassil? IMO if they need a new starting zone, they can't be a sub race.
Yes, that kills 90% of sub races people float. No, I don't care.