No idea, really - I mean it's a Horrific Vision of what N'Zoth wants, so probably Velen is dead, but it's also not real. Alleria in the Horrific Vision would be very powerful, yes; but since it's more an existential nightmare it doesn't really matter in the final accounting. The same is true of Thrall in the Horrific Vision of Orgrimmar, as he was able to seemingly kill almost every other Horde leader in the Council to rule supreme over a tainted Ogrimmar (the only exception being Geya'rah, the last of the Mag'har in the Vision).