I don't have any examples when it comes to novels. But I do have some when it comes to games. The most simple one being Total War Warhammer 3. You open a new character by clearing the game in a certain way. Why not do a similar thing for MMO? To open a hidden class, you have to level up, for example, warrior to lvl 90, and do certain thing on a new character.
The two novel I think of instantly when it come to Vrmmorpg are Overgeared and The legendary moonlight sculptor, with both having hidden class at the start. The hidden class are not only for the mc and other players can gain some.
But I understand the part with plot armor.