I think the best situation is to start with a fresh server. THEN after about 6 to 9 months allow for transfers from Era servers, and maybe even have paid level boosts to help make it profitable for Blizzard. They don’t work for free. Possibly limit level boosts to 1 per account every 6 months to help keep the world populated?
My basic idea on a Classic environment was spelled out here (warning, long post):
A Future Vision of Classic (Original, TBC, and WotLK)