As I previously mentioned, there are a plethora of ways to ‘fix’ thieves (beyond leaving them out of the game entirely). So I’ll show case a few, some from other versions of the game, and others from Bloggers or my own ideas.
Specialization is the Key
2nd Edition AD&D had a great idea to help keep all thieves from being generic, which was that instead of everyone having the same advancement chart, you got a lump of skill points and split them among your skills as you saw fit. When you leveled, you got even more. This had the disadvantage of requiring a DM to generate scores for NPC thieves at higher level, but had the advantage of both making thieves feel different and making certain you had a good chance with at least one or two skills at first level.
So, to turn that into ACKS terms, all characters start with something like this:
Open Locks: 20+
Find Traps: 20+
Remove Traps: 20+
Pick Pockets: 20+
Move Silently: 20+
Hear Noise: 18+
First level characters get 25 discretionary points to divide among their abilities as they see fit (perhaps no more than 6 in any one ability though).
Each level after the first characters gain 8 additional points to spend on their abilities (no more than 3 can be spent on any one ability).
Read Languages and Read Scrolls operate as they do in the book without change.
The problem with this method is what happens with custom classes? ACKS Player’s Companion includes rules for creating your own classes, which would allow you to create variant classes that might have some thief abilities but not the full set. What do you do then?
If a class has at least 5 thief abilities, then I think they could use the above as well. Just they would only start with 12 points and gain 4 each level.
In the case of something like the Assassin with only two actual thief abilities I’d probably just stick to the chart as is. So basically, if there are less than 5 abilities, just use the standard scores listed in the book.
This rule could easily be combined with my first option as well (just like 2nd Edition did), so attributes and race could modify the abilities as well.