I have some questions :) These don't really have to do with plot building but certainly I can throw a few ideas out at the end. :P

What type of act qualifies as bad enough to be reincarnated? Would it be based off of breaking laws like murder and crime? Or Biblical sinning; Adultrey, lies, gluttony, greed, pride, wrath, et cetera?

Through reincarnation to they keep their same personality? Maybe even their same appearance? Though I think it would be cool to show a stream of how your appearance changed between the different lifetimes? Even the possibility of changing genders through the reincarnation. As your soul would be attached to the new body but the chromosomes through the process of pregnancy might change, though has a strong biased towards the original gender of the host? And the only thing that remains the same is the personality of the person?

Now, onto an idea:

Having returned as a reincarnation you have been given the opportunity to right the wrongs of your past life. Though the degree of the sin/crime determines the atonement one must do to break free of your worldly chains. The greater of sinners/criminals have only one option. Join a secret task force of paladins that aid the world through the shadows. Mostly against the Shades, capturing them and bringing them to a secure facility for detainment; knowing killing the shades would only offer them a chance of reincarnation. CAN EXPAND IF YOU LIKE THE IDEA