Essentially like in Cyberpunk 2077, a game mechanic, or specific class, that is able to buy/craft/find augments to tweak their stats or give special abilities. Could be something of a social outcast until they are able to get an upgrade to make them appear more human. You can only equip ONE special ability at a time. Takes damage from both water and poison.
Upgrade ideas (Only one equipable at a time):
Heelies. Ability to have your feet turn into skates to move like the courier. Still like being on ice.
Transformer. Ability to straight up turn into a car. Taking damage effects your normal health.
Super senses. Bullet time, slo-mo with added accuracy. Long cooldown.
Targeting system. For a limited time your gun auto-targets anybody who is hostile to you. No aiming required.
Hologram skin. Give yourself the appearance of another class, gangster, mobster, cop, ect. Illusion breaks when anyone makes contact with you like the assassins' cloak.
Tank armor. Deploys super armor, slows you down but gives you substantial damage resistance.
Arm gun. Like the mech pilot but with less ammo. Much smaller, acts more like a pistol. Maybe it can be charged like megaman.
Scatterbombs. Deploys several small bombs in an area around you. They don't do much damage but they knock people around like crazy.
Signal scrambler. An antenna pops out of your head that send electronics haywire. Noticeable by the cops.
Paranoia signal. An antenna pops out of your head that makes some people go a little crazy, doesn't make aligned people attack each other but will probably cause other NPCs to start fighting. Still noticeable by the cops.
Suicide bomb. Explode your own body when you die. Passive ability. Probably not super useful in singleplayer...
Grand finale. Explode your own body like a big bomb, only able to do it once, will never be able to be done again. Kills you spectacularly. Probably not great to do in singleplayer. Has to be manually initiated while still alive.