Long time lurker, first time poster. Now before I get any snap-reaction flak for this, hear me out.
While I see that Horde will already be getting two extra druid options in Highmountain Tauren (which, aside from just being Tauren, I don't understand why they are getting druid), and Zandalari Trolls (which I fully support due to Loa connections), I'd like to make the case for Nightbourne druids.
The first and strongest point I'll make is that the entirety of the Nightfallen storyline in Suramar is about striking a balance between nature and arcane energies to use the arcan'dor to restore the Nightfallen people from their withering. I don't see how this doesn't perfectly align with Balance Druids.
Secondly: The Nightbourne were Night Elven, more than that, a major part of the Night Elf empire prior to their isolation. It's kind of odd that, at the very least, the knowledge of druidism would completely die out. High/Blood Elves who were exiled from Night Elven society specifically were so because they rehected druidism and delved into the arcane, while Nightbourne seemed to feed off arcane out of necessity and desparation.
Lastly: I feel that it's sort of a waste to add extra races if you aren't going to use them as vessels to give us some class combos we haven't seen. As of right now, we're not getting any "interesting" class/race combos with BfA, which feels strange in an expansion where the Allied Race system is a major feature.
I feel at least one "oddball" class option for these new Allied races will help distinguish them as more than a "reskin" which is what the prevailing complaint about them seems to be.