OSR thieves aren't normal, mundane burglars. They're goddamn ninjas.
In 3.x terms, Hide in Shadows has a lot more in common with Hide in Plain Sight than the Hide skill. Hide in Shadows, on a successful roll in shadows (note: does not require actual cover, just bad lighting) is "invisibility until you move."
Move Silently completely overrides Hear Noises. This isn't "I got a 25 on my Move Silently, I sure hope the dragon rolls low". This is quieter than a gliding owl, quieter than the sound of a cat's footfall (incidentally, not all that quiet in my experience). This is absolute silence like inside a silence spell. In sum, magical silence.
Exhibit 3: Climb Walls. You're telling me that this guy can climb a vertical, sheer (ie, perfectly flat) surface at a foot per second, with no gear. Slab climbing is hard, even with sticky rubber shoes and other specialist equipment and lots of training. Like "typically not climbable from the ground up without pre-drilled holes" hard.
Ability to read any language at 4th and to use literal magic scrolls like a wizard at 10th also seem a bit beyond the ken of normal men, no? The d4 HD also points to a life of seclusion and introspection rather than a hard life in the gutters and alleys.
Incidentally, that all of these abilities are semi-magical is part of why I'm opposed to adding mundane gear to support them. Wearing extra-padded socks isn't going to make a difference when you're trying to move this quietly; true silence comes from within, from slowing your heart rate and controlling your breathing and being One-With-The-Void.
Backstab and Hear Noises are alright. They're neither clearly supernatural nor clearly natural, are useful in play, core to the class, et cetera. I feel no need to either explain or alter them.
And then we also have the crappy, mundane abilities like opening locks and finding and disarming traps and picking pockets, which is just the worst of the bunch. The vast majority of uses of pick pockets, in any campaign, will inevitably be either 1) picking pockets of random NPCs for a pittance (and a DM headache) by clueless but excited players, or 2) stealing stuff from one's fellow party members. Meanwhile, opening locks and trapfinding are regarded as the thief's core abilities, but are actually rather boring, quite replaceable with mundane gear and livestock (nevermind magic), and have the stupid one-attempt-per-level rules.
It seems you've been living two lives, Mr. Thievingston. In one, you have magical powers modern science cannot explain. In the other, your primary advantage over a common locksmith is that you are willing to risk life and limb in search of treasure, though you are arguably less skilled for most of your career and very easily discouraged.
One of these lives is interesting, exciting, and potentially fun; the other is not.
So here are some thoughts, from the ACKS side:
- Normal people can already take Alertness to boost their Hear Noises from 18+ to 14+. What if Trapfinding were a proficiency that boosted your Find Traps from 18+ (which everyone can already do) to 14+ and granted a roll to Find Traps without searching whenever you're in the target zone of one? Much more exciting than searching systematically. Make it a class prof for Thieves, Explorers, Assassins, Fighters, and dwarves, send those guys in front, alleviate Search Paralysis but at some risk to the scouts... And then we drop Find Traps from thieves, freeing up a slot for Other Fun Stuff
- Likewise Open Locks. This should be a general proficiency with at most 18+/14+/10+ for success. I can pick (shitty) locks, and I sure didn't burn a proficiency on it. When you lug a locked treasure chest back to town, you should be able to hire a civilian locksmith to open it who isn't also a ninja master, but if you or a henchman have Lockpicking then you can try to crack it yourself in the dungeon (with failures taking extra time, and a max of 3 turns in a row spent working on it before you have to take a break, just like listening for noises)
- Remove Traps: Don't build shitty traps that can't be neutralized or avoided by reasonably clever play. Spike the portcullises open, put wax in the poison dart holes, rig a rope over the pit, put a rag over the insanity mist sprayer, a mirror in front of the disintegrator ray, and so forth. Then get rid of Remove Traps.
- Get rid of Pick Pockets, forget it ever existed.
- Give thieves some fun and useful abilities. Ideally fun stuff which is also 1) party support rather than personal power, and 2) in keeping with the ninja pseudomagic.
- From the ACKSPC, the Barbarian's "Naturally Stealthy" ability makes so much sense for thieves that it hurts. This provides some degree of mundane stealth as a backup for when the ninja magic fails. Even when you're not being spooky-quiet, you're just not as noisy as the guy in plate.
- As the canonical monk-ish class, the Mystic also has some neat stuff we can steal (hey, we're thieves, it's what we do).
- Command of Voice / Mystic Aura is sort of the stereotypical hypnotic pattern thing
- Speed of Thought / Combat Reflexes makes decent sense
- Wholeness of Body / Poison Immunity plays well with both early-game dungeoneering and domain-level assassination operations. Potentially really strong, though. Separate into "ability to manufacture poisons and poison a weapon without risking injuring self" and "immunity to natural environmental poisons" available later?
- Perceive Intentions is great for a party face or scout and retains its value into the high levels, but is notable among social abilities because it doesn't directly make the user into the face.
- Strength of Spirit / Fear Immunity fits well, but honestly what triggers fear other than dragons and magic? Is mummy paralysis fear?
- Plenty of other useful things we could do. Some of these might make more sense as class proficiencies.
- Disguise? Fitting, but it creates the same risk of party-separation that darkvision on a thief does.
- Skulking, like Naturally Stealthy, softens the pain of the low levels. It was regarded as basically mandatory by our thieves in previous campaigns anyway.
- Endurance, for all that it is not exciting, is pretty suitable for an aescetic.
- Familiar: not just for wizards anymore! What sort of competent goatee-stroking spymaster doesn't have a pet monkey or something that can go where he cannot and hear what cannot be heard by men? Or the assassin with his pet viper or raven, or an animate shadow or suchlike.
- A Shadowdancery "shadow step" short-range teleportation ability would be thematic and expand on the mobility theme presented with Climb Walls. Would also allow thieves to shift between the party rear and the vanguard in the standard Dungeon Phalanx configuration. Honestly you could go full-on Shadowdancer mode with a thief rework and it would probably kick ass.
- Some sort of unarmed fighting style bonuses
- Detect Treasure as a spell-like ability would be pretty amusing, very thiefy but not very ninjaesque, and super-useful for generating intelligence in the dungeon.
- Some sort of 'low profile' ability that lets you evade notice during social situations, searches, &c. Less overt Disguise, more like faceless in a crowd. Sort of like Explorer wilderness evasion, but for civilized areas?
In closing: this sort of thief does not model all thieves of literature. But it could be a lot of fun, and deliver on the original promise of some of the core OSR thief abilities while maintaining the thief's role as dungeon and city support+offense (complementing the cleric's dungeon and city support+defense).