Just think about it, wouldn't it be amazing to play as a void-shrouded demon hunter with a meta form similar to the old black Burning Crusade model?
Another issue this would solve is the long grind required to get these races from 20-110, as demon hunters start in Legion content. Northrend and Outland are especially terrible for leveling, especially as Horde, apart from maybe Zangarmarsh.
As a side note, I'm curious if any devs have played through Howling Fjord as Horde, which I found to be especially terrible. The zone is one of the best in the game for Alliance in terms of gameplay and story-rich questing, but Horde get a bunch of slow escort quests, low spawn rate BS, lots of running back and forth, and an aircraft bombing mission that doesn't even give credit for half the kill targets. Obviously revamping these zones would cost a raid tier so to speak, but what they could do is increase the experience from the slow quests to make up for the time spent.
And enabling demon hunters for current and future allied races would alleviate some of the grind involved in unlocking the transmog sets. Justification could be that the demon hunters decided to train people from other races to wield demonic magic to fight Azeroth's enemies.