In some ways I think 3e was the first edition to really have a nine alignment system (or perhaps a ten alignment system). In the editions before it, I seem to recall that it was extremely commonplace to couche alignment, even of deities, with "with X tendancies" .
For one thing, I think it could be useful to bring back True Neutral as an alignment, accompanied by four new alignments: True Good, True Evil, True Law, and True Chaos. These alignments would be limited to supernatural embodiments of alignment. You could add descriptors to these to describe other tendancies, but supernatural alignmet paragons only embody one alignment, not two. So among the fiends, devils would be Lawful True Evil, yugoloths True Evil, and demons Chaotic True Evil. Only possessing a True alignment should be absolute limit your actions and motivations in some way. The alignments or mortals, on the other hand, should always be pliable and subject to change, though not without consequence. To be fully affected by powers like holy word, it shoud not be enough to be evil, you should have to be consorting with a True Evil creature in some fashion (if you pray to evil gods, or do their bidding in some way, et cetera). Or something like that. Definitely a module. (With this system, I think you would get something like... 26 alignments? (True Good, True Evil, True Neutral, True Lawful, True Chaotic, Lawful True Good, Chaotic True Good, Lawful True Evil, Chaotic True Evil, Good True Neutral, Evil True Neutral, Lawful True Neutral, Chaotic True Neutral, Good True Lawful, Evil True Lawful, Good True Chaotic, Evil True Chaotic, Lawful Good, Neutral Good, Chaotic Good, Lawful Neutral, Neutral, Chaotic Neutral, Lawful Evil, Neutral Evil, Chaotic Evil))
On a slightly different note, perhaps one could think of Lawful Good as pursuing the good based on certain rules (like Deotology or Utilitarianism), Neutral Good on good qualities (Virtue Theory) and Chaotic Good as moral pluralism (Russelian morality).