The way that 5e works, each class has a sub class that dictates a heap of class features. The question here is generally, can a character of a specific class take levels in that same class in order to gain access to the low level features of another sub-class.
The best example of this would either be the wizard or the cleric taking additional levels of wizard/cleric in order to gain access to another domain or school.
Like can a Life cleric take levels of Storm cleric or similar?
If this were allowed, what would the effects be on Maximum spell level and other class level based abilities?