It seems like it doesn't make sense to think of the eight functions as entirely separate. Fe and Fi have overlap, Ti/Te, Ni/Ne, Si/Se.
So I propose that there are only four functions (not that this is completely new or anything). Thinking, Feeling, Sensing, and Intuition. There is a spectrum of Introversion to Extroversion for each function. It should not be thought of as binary.
In most cases, people significantly prefer one direction for each function, but there's no reason this should be true in all cases. It does make sense to me that Introversion in one P function leads to Extroversion in the other (same for J functions). But it's never going to be 100% Ni/0% Ne and 100% Se/0% Si. Thinking of Ne and Ni as separate ideas just seems misleading to me (true for all functions).