Personally, I’d like to be able to “Upgrade Character” on the Character login screen, which would upgrade from WoW Classic to TBC Classic.
They can do this without launching “new” servers. Additionally when creating new Classic characters, you can select whether to create them as WoW Classic or TBC Classic, but always have the option to upgrade.
This can allow for active players to decide to upgrade to experience the TBC Classic pre-patch, without having any additional consequences for inactive players. It also allows for players or guilds in the future to progress through all content, if they so desire.