Daevinski

Sure, 100% optional. I'm aware that some people prefer to keep as it is. The suggestion is more for the people who want make things a little bit harder.

I'm not sure about the technical aspects and possibilities... but speaking ideas only... well, the simplest thing I can think of is the character losing the cop status (and maybe moving to off-duty clothing, losing access to the apartment and having only one pistol in the inventory) after one or two warnings (maybe customizable) via radio ~ in case of shooting unarmed/surrendered civilians or other cops ~ and then the Wanted System being triggered whenever another cop sees you... and once you're Busted or Wasted you lose the current character permanently.
 
This way you still can keep playing for fun and going on rampages as long as you can survive of course.