Those are titles, a Merc or Bounty Hunter can easily be a Mage, a little more "street-wise" but a mage in the end.
In general, Assassin (Spec) is someone who will try to kill you before you get to see them (regardless if they are a hired killer, a bounty hunter or mercenary), an Outlaw Rogue are those that we see in movies and other media where they are agile fighters that fight dirty (Pocket sand, hidden gun, etc...), that's why the spec have a lot of pirates vibe, while Subtlety is more about not being noticed, like spies, you can say that most SI:7 would fit this bill.
But in general, a Void Elf, can be any of those, the same idea of "using the void to mask yourself" work for any of them, Assassins use it to slip through the enemies and poison their food or just make a void shift and teleport behind their target, the Subtlety would use it mostly just to sneak past, create distractions and confuse their enemies while they do what they gotta do (get info, steal stuff, etc...), and finally you got the Outlaw, that would use the void mostly as a dirty trick, obscure the enemy vision, use small "void portals" to summon stuff (like their pistol, when the enemy think they are "unarmed"), or just go full Pirate with it and use the void like how Alleria used it in the battle for Lordaeron, but instead of appearing in the mid of the battlefield, appear in the mid of an enemy ship with your crew.
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Well we do have Void users in the Old Gods forces that in game work just like Paladins but with purple effects (if i remember correctly, we see them in Deepholm)
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I think it would be weird if the Void Elves get Astromancer and not NB, i mean... they are the second race that we know that concentrate a lot on astral stuff, the first being the Orcs of Shadowmoon Valley (though i'm not sure if that count as much, as what they though was a star was a Naaru, so yeah, they might not be the best in that area)