Previously, I mentioned that the reason that thieves suck in my games is that there are no traps, and they aren't good in combat. I don't want to add traps, so I must change the circumstances of combat.
The consensus among my players is that dungeoneering combat is largely "solved" by the Dungeon Phalanx - a formation of ranked fighters with plate, shield, and spears, supported by a wizard or two and a cleric in the rear. The phalanx is very strong in melee in hallways, and when met in open rooms by a foe with superior numbers, it tends to hold the doorway through which it entered and let the wizards do the heavy lifting.
But, there is little place in it for thieves, assassins, explorers, and other lightweights who aren't packing sleep. If placed out in front of the phalanx, the expectation is that they will be cut to ribbons, and can't retreat through the shield wall. If placed in the rear, their targets for archery are typically restricted to those already engaged in melee, and as a consequence Precise Shooting becomes a proficiency tax and they still won't hit anything because -4 is brutal.
There are a couple of ways to address this.
One potential fix is suggested by Jeff Rients, in his old post on miniatures scale, where he cites an early source suggesting that back when, a 1" square on the mat was typically ~3 feet instead of 5. This lets you put three guys in each rank... or leave a hole in the middle for your thieves to retreat through. Ultimately, though, this increases the concentration of firepower possible in the front of the phalanx, making it stronger in its typical combat situation (though massed melee humanoid foes probably benefit similar from this change), so I'm not sure it solves the problem. Making some hallways 20' wide instead of 10' would have similar effects, stretching the frontline.
Another option is area attacks, which wreck close formations. Unfortunately, area attacks also wreck thieves, who have few HP and no Evasion-equivalent in ACKS (Blast and Breath is actually their weakest save).
Probably the correct option is larger open spaces and much more skirmishy enemies, with archers, hit-and-run attacks from the sides and behind, and ambushes along avenues of retreat. Basically, Tucker's Kobolds. Notably, one of the main failures of the Phalanx in previous campaigns was under archers in a very large room. This was also possibly the only time a backstabber (in this case a nightblade) was presented with a golden opportunity to make use of that ability in that campaign. So that's a dungeon-design and monster management problem on my end - smart, prepared foes unwilling to just engage in a slugfest with the phalanx place a premium on surprise and ability to engage at range, and larger rooms give thieves some room to maneuver, hide in, and shoot from. Thieves and assassins also excel against intelligent opponents standing guard with an alarm system (gong, war-horn, whatever), who need to be eliminated quietly before they can trigger it (though sleep can also solve this problem).
Clarifications on the nature of light and shadows might also help. I believe that the rules intend for Hide in Shadows to be usable in the fringe-areas of the zones of shadowy illumination emitted by torches and lanterns, though this is never made explicit. This provides a relatively safe place out in front of the party for thieves and assassins, from which they can surprise enemies (because they aren't in the bright zone), in which they can hide, and from which they can retreat to behind the party's front line.
In terms of mechanical fixes to thieves themselves... It probably wouldn't hurt to trade away a couple of rarely-used skills (say, Find Traps, Remove Traps, and Picking Pockets in my game) for some subset of Precise Shooting, Weapon Finesse, Swashbuckling, Skirmishing, Combat Reflexes, Acrobatics, and Skulking. Seriously, Picking Pockets is not even close to as useful as basically any combat-oriented class proficiency. Even Weapon Focus triggers about every 20 attacks, so maybe once every session or two, whereas Picking Pockets is useful basically never. I'm more than willing to let thieves continue using Picking Pockets at its nominal value the one time per campaign it actually comes up (and for hijinks, just standardize all hijinks into a single uniform throw by level, a la Magic Research Throw, but modified by Dex. Maybe with a new class proficiency like Magical Engineering that grants a bonus to hijinks). The thief suffers from a proficiency tax problem to be combat-effective, and frankly being in melee with d4 HD and AC ~5, even with all of those proficiencies, is not exactly low-risk.
Honestly, the correct solution is probably HD 1, Fighting 1, Thief 3. Thief 2 is just so bad that even at 6 skills, Fighting 2 / HD 1 / Thief 1 beats the crap out of Fighting 1 / HD 1 / Thief 2 for not that much more XP. So we're doing it this way instead. This costs 1700 XP, right on par with Assassin (great, advances my goal of reducing XP divergence slightly between classes), and ACKS' class-points system be damned. For skills, you pick up Hide, Move Silently, Backstab, Climb Walls, Hear Noises, Open Locks (I'm on the fence about Open Locks...), Read Languages at 4th, Read Scrolls at 10th (together count as 1), and then three class profs, probably Precise Shooting, Weapon Finesse, aaaand... Swashbuckling? At low levels, Precise Shooting and Weapon Finesse mean that you can actually attack and hit, and Swashbuckling means you might even survive it. At high levels, Precise Shooting and Weapon Finesse fade in importance (Swashbuckling somewhat less so because it scales slowly), but your stealth skills pick up the slack as their success rate rises.
If you wanted to get really crazy, you could even change up those proficiency-sets like Barbarian does. So you might have Merry Robber with Precise Shooting, Swashbuckling, Alertness, and green tights, Rake or Bravo or Hoodlum with Weapon Finesse (or Fighting Style: Two Weapons, for main gauche), Swashbuckling, Combat Reflexes, and a big hat, Footpad or Cosher with Skulking, Skirmishing, Weapon Finesse, and an ugly brown medieval hood...