My game is called "Living in the Alleys", for those who haven't seen my other post. It centers around the characters, who are thieves, and are tasked with stealing various treasures. I want to find a way to give the characters some sort of special abilities that will make them different from normal npcs. These abilities should preferably be thief-like. I want some sort of idea of what you might find interesting.

The two quickest ways I can think of for handling this are...

1) The characters are the thieves, everyone else are law abiding citizens, the authorities, fences for stolen goods, strange encountered creates...basically everything else in the world that isn't a thief.

2) The characters gain some kind of hero points that allow them to make rerolls or give them other bonuses to their assortment of abilities.

How do you see the character interacting with the world? Tell us a bit more so we can give you some more focused advice.

Perhaps instead of having every player-character be able to do everything thief-like, you could make them believable by having the players choose only one or two specialties.  In this way, players have incentive to work together in order to conquer complex tasks.  Some of these specialties could be: breaking into a safe, disabling security systems, plain old breaking and entering, disabling locks, ambush (stealth, weapons and tactics), quick talking (confusing, fooling, charming, or lying to others effectively), hiding (camouflage, disguise, hiding in plain sight, poker face), or maybe someone who can jury rig anything.

I like your ideas. The setting is in a medieval-style kingdom, called Kasmoth. The thieves are actually oppressed citizens, that have decided to retaliate against the corrupt aristocrats and nobility. I forgot to mention that I did create different types of thieves, called Archetypes. There are three Archetypes: Thug, the muscle and combat specialist; Charlatan, a master of deceit and espionage; and the Snoop, specializing in stealth and infiltration. I though of creating a system where the characters can purchase abilities from a list as they gain experience points; but there are affinity abilities, meaning that a Thug will have better chances at learning some abilities than others. If your interested in knowing more, check my other post "Living in the Alleys".

Abilities are a much harder, much more risky feature of game design than I'd thought, as we discovered here in some very intense discussions a few years ago.

You already have archetype (class) based character distinction. Is there any reason for a character of a given archetype not to receive any and all relevant abilities for it? And is there any reason for a character of a given archetype to have game-mechanical abilities which are not about that archetype?

Given what you've presented of your game materials, and speaking only for myself, I think I'd be happiest playing any of the archetypes - let's say a Thug to keep this paragraph readable - and getting any and all Thuggery abilities you can imagine as a game designer. Then we discover through play what kind of a thug he is, by seeing how I as a player bring those abilities to bear, whether in teamwork with my heist-partners or possibly against them for some reason.

Having yet more abilities to add or choose among strikes me as distracting from that essentially basic and fun purpose of play. It also strikes as recoiling into a comfort zone well-established by 1990s games, in which constant addition to and upgrades of one's "powers" is somehow supposed to be fun even though it makes no sense for the basic concepts of setting and genre for that game.

Best, Ron