The current karma system seems a bit on the silly side, and I understand you guys are interested in replacing it, so I'm suggesting something akin to the really old alignment system ala Elric of Melniboné.
Basically, have three "types" of alignment: Lawful, Neutral and Chaotic.
Chaotic would be obvious, as this would be robbers, trespassers, random murderers etc. People who exemplify chaotic behaviour.
Lawful is a hard one, but my idea of it would be to have it be people who kill random murderers, and maybe a few other ungrindable things. It's meant to be harder to be lawful, especially since the only kind've justice we'll really have is frontier justice, where the punishment is determined for the crime on an individual basis and from the emotions of the people involved.
Neutral would be your inbetweens and people just starting out, people who either CHOOSE not to go lawful and chaotic or are just stuck there from their general actions.
It needs quite a bit of refining, of course, but it'll probably fit the mood of the game a bit more. Maybe have certain crafts and skills open up to you based on whether you're neutral, lawful or chaotic and have them disappear if you slip from your alignment.