As a big fan of the Arrow tv series, I've played a vigilante style character and it worked out really well. Lots of fun in combat and great rp. No crazy uniforms but you DO need some calling cards or a signature.
1. Code of Honor is important. Who or what will you allow to be collateral damage? If you are a anti-corporate crusader, are you fine with a wageslave janitor or secretary getting caught in an explosion you rigged to sabotage something? If you go after a go-gang hideout, are the "old ladies" fair game?
2. In an age of gel rounds and stun bolts, do you have to kill to get your point across? Are your kills clean or are they just disguised torture?
3. Targets are often a product of your background, as someone else pointed out. With corporate police contracts, if you are poor or meta you can bet your 'hood gets no coverage or you treated like scum by badge carrying thugs. That means you may have a long history of taking things into your own hands. Maybe you were a rookie Lone Star cop who joined after watching too many trid shows and found the reality too distasteful. Maybe your family had trouble paying protection to the local Yaks and now you are out to make them pay.
4. Relationship with the law? Do you cooperate with them? Got a disillusioned detective contact who slides you info on targets or hires you for low pay runs against scumbag criminals?
5. Crime pays. What do you do with the proceeds of crime? Say you hit a gang that's been running drugs for the Triads. Sure, you take the money and guns for yourself but what about the drugs? Destroy them? Sell them for more money to fund your operations? What does your crew want to do?
This can be a really fun character to play at a street-level where you are hungry for resources and shit can be reallllllly personal.