This, right here. Anyone who’s played the original Warcraft III games knows Illidan didn’t give a %^&# about the faction conflict between Alliance and Horde, even before being banished. Why would he? It’s short-sighted and just plain stupid to fight over a forest or a gold mine when there’s an enemy seeking global annihilation.
In the context of BfA, I’d like to think that the Illidari’s mission, for the most part, stays the same: protect the entire planet, not one nation or another. Faction-wise, that obviously translates to aiding the Champions of Azeroth first and foremost.
That being said, the Old Gods are definitely on the Illidari’s list as well, especially considering Xe’ra herself warned Illidan about the Voidlords. Given Sylvanas’ genocide of Teldrassil, as well as the fact that she’s practically indistinguishable from Arthas at this point, the Banshee Queen is also a very close second after Azshara and N’Zoth.
In many ways, nothing’s really changed–the Alliance/Horde war continues, but those who truly followed Illidan’s path will put petty faction differences aside for the greater good. Given that the Horde themselves seem to be gearing up for a second revolution (Saurfang and Thrall vs. Sylvanas and Nathanos), this remains just as true for Blood Elf demon hunters.