Dragon expansion, Dragon theme, but I don’t see anything involving the Nether Dragons anywhere. It would be cool if they were involved somewhere in the campaign. I understand they’re in Outland but are they going to stay there during the entire expansion about and of Dragons?
And to be clear, making the Netherwing mounts part of the Dragonriding mechanics doesn’t exactly count. I hope I didn’t miss/forget some important detail, as far as I’m aware they may have only been mentioned?
Yeah you’re asking me, man. I’m not sure how they re-introduce Sabellian and don’t bring in the Nether Dragons in some way, shape or form.
Either they can’t hear the call of the Dragon Isles because their mutations or, just like some of the Blue dragons their taking their sweet time arriving at the Isles.
(But given that Sabellian didn’t have him as part of his flight, Blizz may not even consider them part of the flight so just left them out.)
They arent relevant, they live in outlands.
Possible they considered it safer on Outland concerning how the Blue Dragonflight and Twilight’s Hammer welcomed them. 
Probably tha few that remain are in Outland.