I love Classic, and I even prefer it over Live nowadays, but I have a problem with it: it lacks stability.
By stability, I mean having a clear direction for the game. Right now there are many different versions, and no one really knows what is planned or where it is going. There are some guesses, but very few official plans. For me, this uncertainty takes away some of the fun.
My suggestion is a seasonal Classic experience.
This would mean launching a fresh Classic server every year, adding an expansion each year until it reaches WotLK. When WotLK has been running for a year, the seasonal server would end and all characters would be migrated to three eternal realms, one for Classic, one for TBC, and one for WotLK. Whenever an expansion is released on the seasonal server, players would be able to copy their characters onto the matching eternal realm.
At any time, there would be one eternal realm for each version and one seasonal realm with a fresh expansion.
In total, this would mean six servers. Depending on community interest, we might need to multiply that by the available realm types such as PvE, PvP, or RP.
For additional variety, there could also be one or more realms similar to Season of Discovery. However, I would want to see more clarity about their long-term future.
This approach would keep Classic fresh while remaining relatively low-maintenance, especially if expansion launches and server migrations or copies are automated.