Demon hunters are an iffy class atm to add new races to, if you ask me. They were originally created by Illidan to help him fight the Burning Legion; he choose specifically night elves (because they were his kind) and blood elves (because they were loyal to him before they defected to the Horde or Kael’thas and the Burning Legion) to fuse with demons.
But to add new races would mean there would be a sudden need for new demon hunters. Currently, while the Burning Legion still exists, its no longer the huge massive threat it once was with the lost of all its leaders (Archimonde, Kil’jeaden, and especially Sargeras). For a reason for the Illidari to start recruiting from other races, it’d have to be the demons are once again a grave threat.
That being said though, there are two options that would fit new Demon Hunters outside of NE/BEs. As I said in other threads, the two races that could work as new DHs are the Broken (if they were made playable) and orcs.
Broken joined the Illidari, after Akama sided with Illidan, as they shared the common enemy of the Burning Legion. They eventually left him after Illidan locked Akama away, and only rejoined during Legion.
There were orcs left on Draenor after it shattered into Outland, and many joined the service of Illidan. They drank even more demon blood and turned from fel green to blood red.
These two races have served Illidan and the Illidari and would likely be the first two choices of recruits to be new Demon Hunters if there was ever a case where they were needed against a resurgence of the Burning Legion.
Going on that note, Allied races do present the oppurtunity to give races new class choices without disrupting lore too much. Many Allied Races aren’t as lore heavy as base races and have the flexibility to have new stuff added. In addition, since some are the same race as a base race (but with some cultural and/or physical differences), it can be interpreted to give that race a class that the base doesn’t have.
Such was the deal for Kul Tirans getting to be druids and shamans. While they’re the same as Stormwind and Gilnean humans, their culture and backgrounds allow them the lore friendly reasons to be those classes.
So, going on from the Broken, if they were added to the game, they could be an excuse to give us (technically) Draenei warlocks and rogues without disrupting the racial lore. Unlike their non-mutated cousins, the Broken aren’t as Light-focused (regardless of what side you’re on if Broken can actually use the Light or if they’ve been cut off). The trade off for not being priests or paladins can be that Broken players can be rogues or warlocks (which would fit, considering their harsh lifestyles and how they’d be fighting their mortal enemies, the demons)