Blizzard should decide what they are going to do with Vanilla Classic. The player base is currently too thin for both Classic Era and Season of Mastery. I would prefer Season of Mastery over Vanilla Classic because some of the changes in SoM were good and it can be further adjusted to player needs unlike #NoChanges Vanilla.
The problem with Season of Mastery is that it requires a fresh launch each iteration. This should somehow be circumvented or changed because I personally and I doubt I am alone here; I never have enough time to level the classes I want to play with to 60 in time. There should be some way to preserve the leveled characters in some form.
The game could remember the classes and names of your characters for a reincarnation experience gain bonus. They would appear in the character creation screen as ghosts and if you decide to give the character life, you are granted an opportunity to change the name, race and appearance but not class. Leveling the reincarnated characters would be fast, it should cut the leveling time from 1 - 60 by 60% minimum. Perhaps also an overall increase to leveling speed because it takes quite a bit time to get the characters you want to play with to 60.
This way the threshold to start Season of Mastery again would not be so high and you could keep your old characters in a way. The gear reset would be an acceptable loss, because you usually roll another character class anyways. Most important thing however is to not spread the player base too thin, so one PvP server. Adding even a PvE server is risky because it cuts the people in half. Perhaps for the PvE players, there could be an option to put PvE flag on the character for 10 minutes so you can get to your raid or dungeon peacefully, but it would have a cooldown and max usage 3 times a day.