Something along the lines of people starting to dislike you for randomly murdering people, or you having townspeople showering you with rice and flowers because you saved their town from Cthulhu or something would be good.
The system's already there - we have the bounty thing, which makes guards come after you and locals remark on it, and we also have the dynamic dialogue which makes commoners and guards comment on recent events or your equipment etc.
For instance, one of the saddest things I heard as a result of this system was when I was walking through town with my (Flawless) Blade of Woe. A guard says "Wow! Who'd you have to kill to get that blade?" in a joking tone. Not funny.