I think it’s a very logical assumption that The Last Titan will bring with it the return of Illidan, Sargeras, the rest of the Titan Pantheon, and possibly the neo-burning legion that’s been teased along with it. To what degree, and fashion, I couldn’t possibly know.
But it’s a safe prediction to say that if this is going to be WoW’s “Avengers Endgame” to close one chapter before a new one begins, seems reasonable that Illidan would return to rally new and willing members to the Illidari for the final conflict that lies in store.
For the Demon Hunter starting zone: They could either make a new one liked they did for Allied Race Death Knights, or have the Demon Hunter starting zone unchanged, but told through a flashback/memory third person perspective, like many quests already do.
Once that’s finished, the player can actually start in the Fel Hammer or something.
And if not all races, I think Highmountain (lore-wise ex-feltotem) Tauren, Draenei (lore-wise Man’ari) would be a good starting point. Frankly I don’t see Illidan being so picky to deny any able bodies from devoting themselves to the cause if Azeroth needs all the defense she can possibly get. At the end of the day, just like Allied Race DKs, if they new DH races can be introduced in a similar story-motivated fashion, why not right?
