I gave it a lot of thought, and I think I want to try creating an alignment system based around the four temperaments:
-Sanguine: Associated with blood and air, seeks out stimulation and excitement, acts with spontaneity
-Melancholic: Associated with black bile and earth, seeks out security and preservation of tradition, acts with diligence
-Choleric: Associated with yellow bile and fire, seeks out identity and cooperation, lead by idealism and emotion
-Phlegmatic: Associated with phlegm and water, seeks out knowledge and reason, lead by logic and objectivity
[sblock=read more]A creature's nature leans towards one temperament in particular, or might be split between two temperaments. There is much variation within each temperament; this is just a list of general trends and tendencies. All creatures, including fiends, celestials, elementals, fey, and even undead, have a temperament.
Creatures from the inner planes have souls. The soul directs a creature's life force, or spirit, under the guidance of its temperament. It is the source of its free will. Less-developed souls, like those belonging to animals, are more subject to the whims of their temperaments (instincts). Creatures living in the Feywild have especially volatile souls, making them incredibly capricious and spirited, jumping from one temperament to another on a whim. Creatures living on the Shadowfell have especially inert souls, making them listless and apathetic, their attitudes towards anything rarely changing. The nature of an elemental's soul is not well understood, but it tends to be incredibly focused on bringing its temperament to the forefront, which generally matches the associated element.
Creatures from the outer planes lack souls, but their temperaments have an additional component to them: Spiritual polarity. Those residing in the upper planes have temperaments infused with positive energy, while those in the lower planes are imbued with negative energy. My goal for this alignment system was to not have any metaphysical properties that directly correspond to Good or Evil, Right or Wrong. So the effects spiritual energy has on a creature are best-described as a powerful feeling of transcendence and empathy on the positive end, and a total lack of empathy combined with intense selfishness and a desire for dominance on the negative end.
Altruism and sacrifice come naturally to celestial beings, as their positively-charged beings give them a sense of inner completeness and a need to put their spirit's excess energy to use. An angel never feels envy or greed, because it lives in a state of constant spiritual satisfaction. It seeks to improve itself only as a way to better-serve its cause. While it can experience fear, the overwhelming positive energy that flows through its being gives it endless courage to overcome it. Sacrificing oneself for the greater good is not even considered a question; it's merely a calculation of whether doing so is worth it.
Fiends, by contrast, live with the constant belief that they are incomplete, and in a way, they are. Their negatively-charged spirits mean that even the stupidest of mere mortals is more spiritually-whole than they are, and a demon's life is dominated by lust, gluttony, envy, greed, wrath, hubris, and a total disinterest in acting according to anything other than its most basic, depraved instincts. It always wants more, but what it has is never enough because what it really yearns for is the very concept of transcendence that all fiends scoff at. So it dominates whoever it can, accumulates power and takes pleasure in bringing any creature down to its level, or ideally, below it where the fiend can stomp it into the mud. Creatures of the lower planes treasure mortal souls, one more thing they can never have, and many develop ways to steal them. The raw essence of a soul is an excellent resource for spawning mortal minions inherently devoted to furthering the fiend's goals. Gnolls and orcs are two such examples of races born with souls that have been warped by their creators to serve them.
Undead come in two varieties. Some, like zombies, have souls warped by the magic that created them that compel them to follow the commands of the creator, much like fiend-spawned races. Others are in full possession of their souls and are thus free-willed, like liches. All undead are powered by negative spiritual energy, and are thus subject to the same feelings of jealosy, hatred, and longing that characterizes creatures from the lower planes. Undead hate the living, and will stop at nothing to extinguish the radiant lights that taunt them so. However, because undead do still have souls, those that are free-willed and not too powerful (more power = more overwhelmingly negative energy suffusing you) can sometimes manage to temper these feelings, growing merely apathetic rather than envious, or by focusing all their hatred on one specific individual. This is why many intelligent undead feel more at home in the gloom of the Shadowfell. If you come across a ghost in the Feywild, it has almost certainly been driven mad with rage at the overabundance of life, energy, and freedom. Such spirits are very dangerous.
The nature of aberrations is inherently alien and unknowable. Although traces of familiar temperaments and spirituality levels can be found with strong magic, they rarely match the creature's expected behavior. A wicked beholder might register as spiritually-positive, or a cold and rational mind flayer as sanguine. For the purposes of classification, it is recommended to eschew magical means and simply assign the being a temperament that best-fits its behavior.[/sblock]