The Empire of Arathor could be handled the same way the Empire of India is handled in CK2 or restoring the Roman Empire.
A special event triggered by a decision for humans (the culture groups) where if you control the original boundaries you can abolish old empire titles (turn them into kingdoms under the new empire?) and create the Empire of Arathor.
Edit: or maintain the original empire titles and gain another one that makes a good role-playable primary. I would suggest keeping the de jure Kingdom of Stormwind/Azeroth out of this, though.
A special event triggered by a decision for humans (the culture groups) where if you control the original boundaries you can abolish old empire titles (turn them into kingdoms under the new empire?) and create the Empire of Arathor.
Edit: or maintain the original empire titles and gain another one that makes a good role-playable primary. I would suggest keeping the de jure Kingdom of Stormwind/Azeroth out of this, though.