Hidden locations to "join" a Subclass?

Allow me to explain. I believe it would be an interesting mechanic that you would have to find “hidden” (or simply hard to find) locations that have an NPC that allows you to become a Subclass. For instance, to join the wizard subclass of Sorcerer, you would have to find a hidden hut in the Enchanted Forest, or perhaps to join the Priests, you would have to find a Church of Chapel. Or, for one of the Warrior Subclasses, perhaps the Stronghold would do. I’m sure you get the idea.

Please leave your thoughts below, thanks!

There could possibly be requirements you have to complete to be allowed to access a certain subclass, like training but not under a mentor.