If I were to do fresh servers I would have fresh servers for each expac. You cannot transfer in or out of the servers till the server reaches the end of it’s phase progression. At the end of it’s phase progression, players will have a choice of transferring their character for a fee or have their character permanently deleted. The serve will then reset fresh again.
This way people get their fresh servers, Blizzard get’s yet another stream of income, and the forums get another thing to complain about. Win/win/win
Bonus: Completing certain challenges on these fresh servers will reward the player with unique “hardcore” titles only available through these servers. The titles persist after character transfer.