Because of my preferences I edged into supernatural here, but hopefully stayed in the spirit of thief.
Stow- Become drastically better at hiding items on your person. Obviously works better with lots of baggy clothing, but even at base you can tell at a glance if there is anywhere on (or in) your body you could hide that thing you are looking at, then get it there in one fluid motion.
Quick draw- kinda the opposite of Stow, allowing you to quickly and efficiently grab any item you have on your person without making mistakes. It is so easy to fumble or grab the wrong thing in a high stress situation; do not underestimate this one.
Disarm- not quite as good as magically calling something to your hand, but you become much better at knocking an unsecured weapon out of the enemy’s hand, learning multiple martial arts techniques with that in mind.
What’s This?- Gain a baseline understanding of how to use any item you have in your hands. At base level of the skill it is only a short verbal explanation like “put into the correct lock to open a door” or “pull trigger when pointed at the enemy to cause harm” or “do not break this bottle or you die” or “no intrinsic use, but valuable to the right merchant” but can get better with time. Real life saver for people raiding tombs or magical laboratories.
You didn’t see anything: People unaffected by your actions are more likely to ignore or forget you. Unlikely to get a guard to let you pass, but taking note of strange men isn’t the maid’s job, and she’d really rather pretend you weren’t there even without magic getting involved
Parkour- for the getaway. Gotta stay, one jump ahead of the lawman.
Roguish Charm- Boost to charisma and interpersonal skills, much better if someone sees common ground with you. For example, fellow thieves or people who get your obscure references to literature and philosophy
Disguise Mastery- A kind of psychometry. Allows the thief to emulate the mannerisms, vocabulary, and even some surface level knowledge of the last person to wear a set of clothes.
Minor Magic- Whatever the weakest tier of Magic is, a thief can access it, but limited to things like snuffing out candles from a short distance away, extremely basic illusions, or mentally befuddling people that are already tired and bored (like night time guards). Most thieves don’t take the skill because it’s an expensive dead end, unless they are planning on transitioning to a more magical class.