To bring this old ass thread into the light courtesy of the two posters above - The Ashbringer is an undead killing sword, we'd need a story to go around the undead for it to make any sort of sense again. It kinda feels like we've exhausted the Scourge and there's not a whole lot of undead left. Sargeras little burning crusade is just a bunch of demons, it doesn't fit.
I don't think we'll ever see the Ashbringer again, and quite frankly I'd prefer it that way. We don't need to be running around with named weapons in WoW lore.
Hell, it even felt wrong to run around with
Archeus, Morgan Ladimore's sword.