I was thinking the other day about how magic items for wizards and magic items for fighters more-or-less mirror those class' native/inherent resource management schemes, and how those resource management schemes tie back to their roles on the Kirk-Spock Axis (I'm going to refer back to that post again; it's a little roundabout but really good, and you should read it). Fighter magic items are typically persistent, passive boosts which don't need to be shepherded carefully, while wizard magic items are typically consumable, and must be managed in a similar way to spells. This consistency continues to encourage fighters to play (relatively) recklessly and wizards relatively cautiously and analytically into the high levels.
This connection between resource management and playstyle got me thinking about thieves again. As I've mentioned before, the thief's sort of "core mechanic", thief skills, are essentially saving throws against certain situations likely to doom the thief and the party. If you fail move silently, you're no worse off than if you'd never rolled it. They're mainly useful (at least at low levels) in "oh god we're all going to die maybe I can roll out of this and sneak away" situations; high-risk, high-reward, with none of the guarantees of the fighter's AC or the wizard's known spells that solve problems outright.
The very boolean nature of thief skills ("I'm either totally invisible or just as visible as a regular guy, and I don't know which") and their general unreliability leaves a lot of players cold, though. These strain suspension of disbelief, and require supernatural explanation, which is unsatisfying.
Likewise, for gamers brought up outside of the D&D tradition, wizards make no sort of sense. Vancian magic is pretty unusual, and a bad fit for many campaign worlds.
So what if we changed things up a bit?
What if we swapped the positions of the thief and the wizard on Roger's Party Axes diagram, placing the thief in the analytical, cautious, clever position opposite the fighter, and the wizard in the "forbidden secrets, great sacrifices, desperate measures" location opposite the cleric's consistant, happy moral magic? And updated these class' mechanics accordingly, of course. What would that look like?
Fighter-vs-thief is, in some ways, a quitessentially Combat-as-Sport vs Combat-as-War axis. The fighter is very good at dealing with the tactical exercise of combat, but not strong outside of it. The thief, on the other hand, is logically strong at avoiding combats and mitigating losses but weak at direct combat. The fighter welcomes the glory of single combat against a worthy opponent; the thief would like nothing better than attacking in the dead of night with superior numbers (or, better yet, stealing the treasure, opening the gate, or otherwise achieving the objective without fighting or being noticed at all). So we have a tactical-vs-strategic conflict here. The fighter is already well-equipped tactically; what does an alternate thief equipped to deal primarily strategically look like?
If we accept that strategy is "putting yourself in a position to win battles and profit from them", then two functions immediately spring to mind for the Strategic Thief: intelligence-gathering and logistics. Logistics is, of course, boring, but we could certainly cherry-pick an ability or two from the venturer and call it done (in a way, the hijinks system as it exists currently serves a logistical support function at the domain level, in that it provides cash to enable other operations).
Intelligence-gathering and public relations, however, are rather more interesting. Abilities like Perceive Intentions, Hear Noises, and Sensing Evil let you avoid fights before they happen, saving valuable resources, or let you get the drop on enemies and win fights more easily. Likewise, map management and operation-planning that brings you to the treasure more quickly reduces the number of risky fights you might have to deal with. The thief is logically well-suited for a scouting role, though historically this is both risky and somewhat boring for the rest of the party. I do not have a great solution to this problem just yet, although I suspect that in ACKS at least the way this plays out is that the thief has to remain within the area of shadowy illumination in front of the party's torch (in order to see anything at all), which requires the party to advance relatively closely behind him.
Another problem with using the thief in an intelligence-gathering / combat avoidance role is that ultimately, his function is diminishing the "fun" of the game, if one assumes that combat is fun rather than deadly. In a game where the enemies are very dangerous, the thief will dominate the fighters; in a game where the enemies are weak, the fighters will tend to overrule the thief. Treasure-sensing abilities would sort of address this and be quite thematic; they give the thief a "we should" piece of intel, rather than being purely "we shouldn't". Maybe an Appraisal ability that lets a thief estimate the value of a magical object or pile of coins instantly. Done directly, treasure detection could strain belief, but such abilities could also be explained as having gathered map fragments and rumors in taverns (HumInt). Making the thief the go-to guy for rumors and partial maps before the expedition might actually make a good mechanic; exchange time and gold for rolls on the Dungeon Intel table, which are modified by thief level, charisma, distance from dungeon, and level of dungeon, with results ranging from false rumors from peasants to maps from reliable veterans who've been there before you and door passwords or other secrets from sages. A more resource-managementy and mechanical concept here (to fill in the gap being left by the wizard) would have the thief gather intel points through Tavern Research rolls, which can then be spent in the dungeon ("consulting your notebook for a few minutes, you deduce that...") to 'cast' effects like Detect Treasure, Find Secret Doors, Align Map Fragment, Locate Shortcut, Know Password, and so forth, Indiana Jones-style. Heck, you could even tie Find Traps into such a system, if you're so inclined as to trap. This also transitions nicely into...
At domain levels, the thief runs a spy network, which he can set to work gathering information on enemy troop dispositions, leader capabilities, realm morales, and so forth. While the fighter is leading troops, the thief's network is sabotaging supply caravans, funding resistance organizations, kidnapping heirs, and otherwise causing trouble. The spy network is a well-established trope, and one which I suspect would be more satisfactory and less... inconsistent with the game's economy than the thieves' guild.
So what does the strategic thief look like? Backstab and hear noises stay in their current forms; they're solid abilities and we want thieves to be somewhat useful in combat. Modifiers to surprise rolls and enemy surprise rolls in the vein of Naturally Stealthy and Combat Reflexes (but improving with level) let the thief scout effectively while making use of (rather than circumventing) ACKS' existing mechanics for stealth and make the thief more consistently sneaky. Some sort of Black Market ability lets the thief provide materiel support (maybe "divide the cost of an item by a number up to your level +1 for purposes of finding it in the market, but multiply the price you have to pay for it by that same factor; usable once per item per market per month"). Finally, a Tavern Research roll mechanic fills out the intelligence-gathering abilities. At domain level, a spy network lets you use a similar mechanic to gather intelligence or perform spooky operations at larger scales; rather than tracking individual ruffians, your spy network is mostly comprised of no-detail "cells" of rookie thieves and common men, which let you perform intelligence-gathering in far-away lands, as well as a few high-level "operatives" who can incite cells to do riskier things.
(Amusingly, this actually removes one of the fighter-thief conflicts present in vanilla ACKS, namely "thieves' guild is looting my town for obscene amounts of money but not actually hurting domain morale so I guess it's OK?")
There is, of course, the question of "whither traps and locks?" Make Lockpicking a general proficiency at 18/14/10 and Trapfinding a class proficiency for fighters, thieves, assassins, &c. (Assassins, incidentally, are mostly unchanged - they replace hide in shadows and move silently with scaling surprise modifiers. Their domain game needs some work I guess)
That's that for the thief. Next post, because this one is quite long enough: wizards.