I suspect that it may be misleading to think of the functions as eight separate things. I've been playing with descriptions that describe extraverted and introverted forms of each function as more like slight variations in application of what can generally be understood as a unified concept.
The idea is roughly that:
Intuition would be to make disparate connections, operate within theories, and to take an attitude of abstraction. To extravert it would be to take this attitude of abstraction primarily with respect to the external world; to introvert would be to take an attitude of abstraction to one's own thoughts.
Sensing would be to focus on concrete, sensory-factual data, and to take an attitude of literality. To extravert it would be to take the literal attitude toward the external world, and to process perceived data; to introvert it would be to take a literal attitude towards one's own thoughts, and to process preserved data.
Thinking would be to logically structure and to eliminate inconsistencies. To extravert it would be to structure the external world and to eliminate inconsistency in the form of systematic inefficiency; to introvert it would be to logically structure one's own thoughts, and to eliminate inconsistency within internalised belief systems.
Feeling would be to process emotions and make decisions thereupon. To extravert it would be to process external emotions (as they are perceived as existing in others) and make decisions in response to this processing; to introvert it would be to process one's own experienced emotions and respond accordingly.
This is basically the same theory as that which consists of Ni, Ne, Si, Se, Ti, Te, Fi and Fe. It's just supposed to better highlight the connection between Ti and Te, Si and Se, and so on - rather than seeing Te, for example, as a draconic enforcer of organisation without a soul, and Ti as the essence of analytic rationality, or Ni as a mystical force and Ne as crazy unrestrained creativity.
I'm wondering if it might make it easier to see how, for example, Ni use might work in a 'Ne user'. An NP might generally prefer to extravert their intuition, but as someone who prefers N generally over S, it's not difficult to imagine them sometimes introverting their abstract approach. It's not even inconsistent that they should do this despite generally preferring a literal attitude to their thoughts. It doesn't necessarily have to be evidence of a 'poorly balanced' personality.
It would also mean, for example, that two types differing only in J/P wouldn't be said to have no functions in common in their top four. Rather, they have their top two functions in common (perhaps in a different order of dominance), but may tend on average to employ them in a slightly different way.
Thoughts? It is a vaguer, less clear-cut description, but what I'm wondering is whether that's a bad thing.