So, here's how I think I'll chart alignment.
First, I need a scale. I was originally thinking of using a scale of 1-100, but that seems too small for me. I don't want just a few actions to rapidly shift alignment; I want a character's alignment to really, really be a product of habitual action. A scale from 1-300 is, I think, about the right size; without testing, I won't know for sure.
Next, I need to decide which ranges are Chaotic, which are Neutral and which are Lawful. I think the 1-50 range will be Chaotic and the 251-300 range will be Lawful, with 51-250 being Neutral.
Then I need a list of activities that move a character along this scale. These should be relatively big things; pickpocketing, for example, doesn't significantly support the cause of Chaos or the downfall of Law. In fact, I'm leaving theft completely off this list since that's such a common activity in D&D in the first place. The actions on this list should actively and significantly build up or tear down the health and order of society and the universe at large. Here's the list I have so far; other item suggestions are welcome.
Freeing the enslaved: +4 per slave freed
Defending or rescuing the helpless: +5 per victim rescued
Bringing the guilty to justice: +10 per HD
Banishing Cthuloid beings: +15 per HD
Moderation: +1 per 100 Gold Pieces given to worthy causes or the less fortunate
Joining and taking the vows of a Lawful order: +50
Necromancy: -10 per HD
Slaving: -2 per slave bought, sold or owned
Slaughtering innocents: -10 per HD
Sparing the guilty: -1 per offense overlooked
Consorting with Cthuloid beings: -10 per HD
Excess: -15 per ostentatious purchase or night of debauchery
Betraying or falling from a Lawful order: -30
You'll notice that Chaotic acts will bring someone towards Chaos more easily than Lawful acts will bring someone close to Law. This is for a few reasons. One is that it is easier to destroy than to build. The other is that Law is a lofty ideal that is difficult to attain and maintain, while Chaos is insidious, tempting, easier and, being the corruption of the ideal, multitudinous in its paths.
Some things I like about this:
This gives a more objective criteria for allowing or barring existing characters from joining Lawful orders: If the +50 from joining an order won't get a candidate into the Lawful range on the alignment scale, they aren't accepted. By a similar token, fallen Paladins (or whatever other Lawful orders boot their members when they mess up) may only undertake a quest to redeem themselves if the -30 from breaking a rule of their order and being expelled doesn't take them outside of the Lawful range; otherwise they won't be allowed to re-join, ever (or, if you want to be nice, until they get into Lawful range again).
This also allows for Paladins and other Lawful types to engage in limited Chaotic activity, so long as they are balancing it out with Lawful acts. This way, they don't have to be super-careful about their actions. A very Lawful Paladin could indulge in a night out on the town with his friends and still be a Paladin. With this system, Paladins don't have to be as strict and grumpy about every little act.
A quirk I purposely built in: buying a slave and setting him free results in a net gain of 2 alignment points. Rescuing a slave and killing the 3HD slaver nets 34 alignment points.
I'm currently planning on all classes starting out at 150 unless their class is inherently Chaotic or Lawful. If a class is Chaotic, the character starts at 50; if the class is Lawful, the character starts at 251.
I realize this system is still rough, dirty and untried; I have no idea what the kinks are that will need to be worked out, but I do like this system and plan to try it out. If you have comments, critiques or suggestions, they are very much appreciated: what to you think of this system?