I mean, until Legion — Arator’s last appearance was Burning Crusade. 
Everything else was just books or comics.
Additionally, there’s been many other significant characters who haven’t appeared in the game previously – that doesn’t really negate their importance.
If anything, that’s precisely a reason as to why they should help establish it further in the game. 
Especially since it’s a faction of sorts that the Horde can take pride in, as is comprised of multiple races of the Horde – rather than just one or two.