This idea came to me when I went to read a fan-fiction on this site, which I can't remember what it was called. The plot, I think, was a rookie cop gets killed and then gets wishes, and one of them was for a system. When I first saw the title I had assumed it was going to be about a system specifically for solving crimes and to make the protagonist a great detective, but it turns out it was your generic gamer system, and he was reincarnating into Marvel or DC. I was basically mad at that point, I felt betrayed in a way. So, I came up with this idea, which is a system that gives rewards and experience based on the cases solved by the MC. I personally believe that this is infinitely more interesting than a boring old normal Gamer System.
It works like this: Quests, or Cases as they're called, are the only way to level up and gain free stat points. This isn't the only reward they give, as they can also give out [Skills] and special [Items]. All these rewards must be something related to a crime series, so things like Kryptonian Physiology are out of the question, but something like Batman's detective skills aren't. [Items] aren't as limited by this as long as they aren't too Fantasy-esque. No health potions, but a Stimpak from Fallout might be applicable. It also includes things like weaponry, i.e. the Dominator from Psycho-Pass, the Lawgiver from Judge Dredd, and the LAPD 2019 Blaster from Blade Runner are completely within the bounds of the System. As long as it was used by those in the criminal justice system, it is able to be acquired through Case Rewards and the "Shop", once unlocked. Could probably come up with a cool name for it, like the Armory or the Quartermaster. Or something else that is close to those, but I digress.
Now, what about the setting? Should it be a world akin to ours, where there are no supernatural powers or entities, or should it be a fictional world, where a random character gains access to wonderous abilities from other dimensions through the System? Both have their pros and cons, but maybe I should be more specific. When I say a fictional world, I don't mean somewhere like Marvel or DC. I'm more referring to a world where there aren't Gods and Demons walking amongst the common folk. Like maybe the world of Psych or The Mentalist, some world where the Police have a certain degree of relevance.