I think what we’ll find is a new DH-capable race needs to be added to Alliance and then both that race and Nightborne (or Zandalari, e.g. the Demoniacs who seem to have been retconned out) can become Demon Hunters. Void and Fel react violently, keeping Void Elves from practicing this art, otherwise we would already have 4 DH races in the game.
Alternatively, they could open up a few more existing races to the class, seeing as there are two Murloc Demon Hunters in Legion. As ridiculous as that is, they are legit DHs, so DH Humans, Green Orcs, and even Gnomes don’t seem so farfetched in comparison.