Players are allowed to keep all characters to encourage replay with new characters classes. However, on level 60 characters the gear is reset to 60 level dungeon equivalent gear. This gear could be items already in the game or Diablo style seasonal rewards.
Seasonal “rewards” would have inferior stats compared to high end items in game but they would have either unique look and/or would have interesting but not overpowered set bonuses or something like that. Point being that stat wise there are better items in game but you would still want to keep the seasonal rewards for some use in the game.
Additionally an experience gain bonuses per 60 level character you already have.
I stated that players would be allowed to keep their old characters, they would just lose the gear on 60 level characters so there would be something to play for in the new SoM season.
if they do a new SoM it would be fresh - Blizz gave free character Transfer within a window and made it clear characters not transferred would be deleted.
It did seem, however, that I was able to transfer by SoM Lock to Mirage Raceway where I have the rest of my toons, which was fair in my view.
So there will be only fresh on any new Season of Mastery
They can do smth Diablo-like season format. Increase %xp boost more and more, cause dropping leveling remove “1st 60 on realm” and disable a lot of game content and low level action.