[Hyperborea] Need some ideas for 'unlocking' subclasses!

I am going to be starting a Hyperborea 2e campaign soon. A hex crawl sandbox set it a post apocalyptic fantasy realm where the players will initially start as adventurers setting forth from the last great city, whose magical wards that protected it from the cataclysm have finally died away. The world beyond has changed significantly and as far as the people of the city are aware, very few survived the cataclysm (this of course will be proven false as they come into contact with the many varied new cultures that have sprung up!)

At start the only available Classes will be Cleric, Fighter, Magician and Thief but I would like to figure out ways that the players can potentially unlock the various subclasses!

I think there should be a flavorful requirement for the Subclasses to become available, both as options for the players to switch their characters to, and for making the new subclasses available as options for hirelings.

I am looking for ideas as to what might be required to open these new options up.

-And here are the subclasses for easy reference-

Fighter: Barbarian, Berserker, Cataphract, Huntsman, Paladin, Ranger, Warlock

Cleric: Druid, Monk, Priest, Runegraver, Shaman

Magician: Cryomancer, Illusionist, Necromancer, Pyromancer, Witch

Thief: Assassin, Bard, Ledgermainist, Purloiner, Scout