On the Ptr void dragons got datamined. Which is nothing bad on its own. BUT, dragon lore at the moment is pretty rare. And another new race of dragons we focus on will take even more lore away from the main dragonflights.
Specially the twillight dragons will probably be forgoten by the writers. The writers abandoned the concept of twillight dragons , which they set up in the novels pre dating Vanilla , to make them generic bad guys and use them to always fill the ,, void ´´when a evil dragon is needed.
Now in Legion ( shamen order hall quest ) and in BFA island expedition quests, we finally see the twillight dragons act as independent flight , with invidual characters.
But with Void dragons we run the danger of giving blizzard new generic evil dragons , which than will get all the focus on them.
The nightmare dragons atleast had their lore already be made before legion. So they were mostly Ignored.
TLDR : Void dragons will be useless or eat up all new dragon lore