Can Bolvar even raise new undead without the sword? Tirion sorta broke it.
He who sees his own doom can better avoid its path. He who sees the doom of others can deliver it.
Maybe Wrathion should have gone to Icecrown and defrosted Bolvar instead of sending Garrosh back in time.
After the fucked up Warlords timeline i doubt that we will get anything in that direction.