Update (07-Jun-16): After some more playtesting, these house rules have been replaced with revised Thievery progressions.
One of the legacies of ACKS is that thief abilities use a per-level throw value while other proficiencies have throw values that are flat and increase by +4 when additional ranks are taken.
The result of this legacy rule is that thief abilities are notoriously unreliable at early levels; and by the time their abilities become reliable, thieves find themselves lackluster in comparison to spellcasters who can cast Find Traps, Knock, and Telekinesis.
More recently, Beragon suggested the following house rules to help mitigate this concern:
Open Locks: Codify the complexity of a lock and assume that the number on the thieves’ skill chart is applied against a high quality lock.
Common lock: +8
Good quality lock: +4
High quality lock: +0
Find Traps: Make this skill operate passively. If actively searching, the character gains a +6 bonus (this is modeled after the keen eyes ability of elves). Essentially, a thief gets up to two chances to find a trap… a passive chance (rolled by the Judge) and an active chance (if the player declares a search).
Remove Traps: Codify the complexity of the trap and assume that the number on the thieves’ skill chart is applied against a very complex trap.
Basic Trap: +8
Tricky Trap: +4
Difficult Trap: +0
Pick Pockets: Allow a thief the option to attempt to snatch an object out of another’s possession without regard to being noticed (i.e. the lowly act of grabbing an old lady’s purse) at a +4 bonus.
Move Silently, Climb Walls: If a thief keeps his encumbrance low – 3 stones or less – he gains a +4 and +2 bonus on these proficiency throws respectively.
Hide in Shadows: If the thief’s clothing blends in with his environment (most common = dark clothes when in dark or shadowy conditions), he gets a +4 bonus to Hide in Shadows.
Hear Noise: No change.
As moorcrys notes, these rules give the thief additional agency without re-writing the class or the rules.
After struggling with thief characters in other old-school games, I’m planning on incorporating these house rules in my (still hypothetical) Astonishing ACKS campaign.