I'm having a difficult time reconciling my world with the new sub-classes presented in the XGTE.
In the PHB, I see two types of sub-classes, Natural Sub-classes - those that are a natural extension of the class, such as the barbarians's Path of the Berserker or the monk's Way of the Open Hand - and Tangential Sub-classes - those that take the class into a new direction, including the fighter's Eldritch Knight and the monk's Way of the Four Elements.
While it seem entirely possible that the abilities natural sub-classes could have arose a natural extension of the character's original training necessary to become a member of his or her class in the first place, tangential sub-classes would require more specialized instruction.
That instruction may come in the form of a divine or warlock patron or else a knowledgeable teacher. In some cases, such as the Eldritch Knight, that teacher need not be a member of the character's class; any wizard could instruct the fighter the ways of wizardry. At other times, as in the case of the Monk's Way of Shadow, a more specialized teacher (an adept in the ways of shadow magic, though not necessary a monk) may be necessary.
This, however, creates some difficulties with regards to world building and player agency. I want my players to have the widest variety of classes and archetypes to choose from, but I also want those choices to make sense within the context of the immediate setting and the world at large.
In some cases, such as the sorcerer, in which the character's sub-class is chosen at first level, there is no disconnect. The character has simple already received the training and knowledge of the principles necessary continue down and develop the entire list of abilities associated with the sub-class.
Classes that choose their sub-class after first level, such as fighter, monk, and paladin, are more problematic. While the player may want his or her character enter one sub-class, the arcane archer, for example, no instructor may be available. While the character could certainly search for a teacher, forcing character to choose their subclass at a specific level sets a time limit, by which time the character must locate such an individual. Alternatively, I could present a plethora of well known trainers easily accessible to the characters; unfortunately this places an uncomfortable amount of stress on the setting and raises the question of why the PCs' heroics are necessary in the first place with so many high-level characters nearby.
So. How to you solve this predicament in your campaigns? How does AL choose to handle it?
(FYI, "I don't care to think about these things and you shouldn't either" isn't the type of answer I'm looking for.)