That doesn't sound like a PrC mechanic, at all. Actually, it sounds more like a 'Theme' - any class can take it, it lets you swap out existing class abilities for 'theme' abilities.
The problem is that sub-classes aren't consistent when it comes to how much of the class is in the base class vs under the sub-class. Each Theme would need to specify how it interacted with each class, individually. (In another thread, I was pushing the pro-5e idea that 5e could handle any concept from prior editions, I guess I might have just found an exception. Annoying.)
How would that work, though? You give up several levels of you main class? That's a big sacrifice for some horseback (or even dragonback) riding lessons.
OK, or feats, sounds more like a feat,