The Dark Side

Mentor: Yes, run! Yes, a Jedi's strength flows from the Force. But beware of the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice.
Hero: Vader... Is the dark side stronger?
Mentor: No, no, no. Quicker, easier, more seductive.
Hero: But how am I to know the good side from the bad?
Mentor: You will know... when you are calm, at peace, passive. A Jedi uses the Force for knowledge and defense, never for attack.

Games, as one famous designer has noted, are a series of interesting choices. One problem with the traditional D&D alignment system is that its a straightjacket, not a choice.

I think this comes from the game's origins as a war game. The players would set up their minis, one side playing the forces of Law and the other the Antithesis of Weal, aka Chaos. Maybe a few neutral critters thrown in for fun. Its like one side using the white checkers and the other the black, or one team being Axis and the other Allies.

This concept was expanded to AD&D, where you really had a spectrum from LG to CE, with CG and LE really being sort of fringe alignments in the middle. In 4E, there's a return to a more linear spectrum.

But alignment doesn't really entail any interesting choices. It might be a code of behavior and a way to enforce a degree of order among some players ("You're Neutral Good! Raping, murdering, and desecrating the corpse of that orc will lose you a level whether the paladin is watching or not!"), but other than that it doesn't do much. AD&D encourages law to some degree -- your hirelings get morale bonuses -- but that only applies to the subset of players who care about employees and the even smaller subset of DMs who impose morale rules BTB.

If I were to tweak the simple alignment system in OD&D or 4E (the spectrum from Law/Good to Chaos/Evil), I'd add a mechanic that gives short term benefits for evil/chaotic acts versus long term benefits for lawful/good acts. I would also hide the numbers from the player so that they don't know how good or evil they are at the moment. Otherwise its easy to game the system. For example, evil acts might regain you HP, give bonuses to hit and damage, or make you more likely to succeed at a check. They'd have very strong benefits for good characters (to increase the temptation), lesser benefits for neutral characters, and small benefits for evil characters. All of these are valuable resources in a pinch, making characters more likely to compromise morals for a short term benefit when they've been pushed to the limit of endurance.

For example, take "fighting dirty." A good character that fights dirty might get 2d6 retain the highest bonus damage added to a roll but take a hit on their alignment. A neutral character that fights dirty gets 1d6 bonus damage added to the roll. An evil character gets nothing, or perhaps 2d6 retain the lowest at most. They need to move on to even more depraved acts at that point, or recover some morality.

You could also award an "action point" or something like that for Evil Acts, with Good characters perhaps getting two (to make the temptation strong).

A good character would see some sort of minor long term benefit. A bonus to saving throws and/or bonus to reaction checks with good and neutral characters would fit easily. You could even throw in minor Paladin-like bennies for very good characters (benefits of a pro evil spell, etc).