To be honest, if there is a sub-class available in game, I'd rather be able to choose it at level-up. As opposed to search around the world to find it. Because if I wanted to try it out, I'd just find a walkthrough to point out the necessary steps to get it, instead of blindly bungling around the world map in search of it.
+1 yeah, I don’t mind one off stunt like that (though from what I understand current iteration leaves some to be desired), but having subclasses unlockable ingame doesn’t sound like best idea. Unless there is a good reason for it, I definitely would prefer for it to be available on level up.
Some for of prestige class would be another matter, but I don’t know if 5e has something of that sort.
