Actually, something similar would be the Wrath of the Lich King expansion for World of Warcraft. The new Death Knight class had a completely separate starting story unlike all of the other races. Perhaps the same will happen for the Revenant class, and the characters that start in the Revenant class would have a completely different story and not the race.
E.g. Revenant character may be a normal Tyrian citizen thrown into the Mists where they learn the Revenant class just like Rytlock did.