We had a couple of ACKS campaigns (both in "generated" fantasy world, not really detailed and with a bunch of B* adventures thrown in), and a second one ended recently, so I decided to share some of what we had. Rules are in no particular order.
1. AC/armor rule (or rather assassin/mage rule). Assasins lost their ability to wear plate, but in return they've received thief abilities in magical ring or chain armor IF its effective encumbrance is the same as leather armor or less (so +1 ringmail or +2 chainmail counts as leather for using thief abilities). Mages got ability to wear and cast in magical hide (becase "if it's zero effective encumbrance then it must be weightless and really cozy", but in fact I just wanted to find a way to use all the randomly generated hide instead of just selling them).
Campaign effect: For some time assassins were the most popular class, skulking in the shadows before the party, but most died to traps/wandering monsters and everything got back to where it was. Still, it made assassins more useful. Some mages died as well because "I'm in armor, I can handle a hit, right?". Less money and XP for the party (they were selling most of magical hide before), still not sure if it's good or bad. More recklessness is always good though.
Next game: Keeping both. More AC only really affects classes which already have high AC so combat effect it small but makes players happier.
2. No natural domain growth, it all evens out, so morale or investment growth must come from SOMEWHERE...and that's usually a neighboring realm or several. And if your neighbor has 30k families in his realm and you're dumping enough money into agricultural investments and getting enough morale to attract 165+ families this month, neighboring realm morale drops to -1 (165 families is what they would lose if they had a -1 morale penalty). If you have more than one neighbor, they would lose population proportionally to their size.
If you attract 330 new families, that neighboring realm would have -2 to morale. -4 if you gain 660, and so on. And if you're dumping a lot of money into agricultural investment every month, their ruler(s) would be very, VERY angry at you and might declare war, or at least won't be friendly if their penalty is mild.
Campaign effect: Well...three duchies and one principality declared war, because players were dumping everything they had from adventuring into realms' growth month after month (and they were also a bit underleveled for their realms' sizes), and that's how campaign actually ended. It was mostly my fault as a Judge because I didn't signal the consequences clearly (I actually wanted it to be a surprise, and it was a bit too surprising I guess, they didn't have enough troops, and after the first defeat with lots of casualties it was clear that the war is lost, and they wanted to just wander the world, which wasn't detailed at all).
Next game: I'd start with a visible threat first, find stuff for them to be more attached to, and next time players plan to start with a single realm with different responsibilities (most likely based on Senate). Still keeping this rule, because it puts an upper limit on domain growth, which isn't arbitrary and is actually fun (I wish I made a photo of players' faces when they realized that they almost depopulated their neighbors). Will probably halve the penalty to neighbors accounting for immigration from other realms, it was probably too harsh. And no surprises, heh.
3. In second campaign I've removed level limits for demihuman classes with max level 13 (and upped it by 1 for others)...but they've lost access to arcane and divine magic, instead getting eldritch (with the HFH XP cost), which explains why humans are ruling the world way better than level limits. Also I gave highlevel demihuman NPCs some NPC-only spells which were basically plot device. Elf queen whispering to PCs a warning about some danger using a subtle wind when they're days away, hard to travel in demihuman realms if you are not invited (-4 to getting lost) because of moving woods/shifting terrain/illusions/etc, and so on, with a huge emphasis on mysterious and magical.
Campaign effect: Players LOVED that. NPC-only spells were really great. Demihuman magical classes were still used by players because eldritch magic offers a lot of choice from both arcane and divine pools, and they've rationalized the lack of NPC-only spells as "only crazy ones will wander off to become adventurers, and now no one will teach them because they're tainted by humans".
Next game: Keeping it. It's cool.
4. Anyone can get ceremonial magic with a general proficiency, getting 1/2 of runemaker progression, or runemaker with 2 proficiencies, or or loremaster progression with 4 proficiencies. Anyone as in, ANYONE.
Campaign effect: It was so easy to make NPCs harder and/or have more abilities ! After all, they can have ANY trinket (and ANY amount of them). Thief got into players' house ? Well he had a chameleon trinket. They've caught him but he got away ? He had a blinding flash trinket. Enemes figured out their ruse ? They had a ceremonialist with an ESP trinket ready. I'm not realy a railroading Judge and prefer to just roll stuff to see what happens, but when I want to push players to some direction or explain why "that" happened after a random roll, it was just really easy to do, because anyone can have magic. For me it became yet another plot device.
But I rarely even had to explain it, players did that themselves ! Never before a "wizard did it" was so easy. Seriously. I made them rare, and when I rolled NPCs they had ceremony proficiency if I also rolled a natural 20 on a separate d20 roll, and only if they are classed, but that still meant that a hamlet can easily have a ceremonialist (14 classed NPCs so 70% chance), or that a Class IV city criminals certainly have several in their ranks. It made magic a bit more ubiquitous...when it was used against players, but still not cheap or affecting the economy, because making trinkets costs money.
Players didn't use it. At all. As they've said "it's like shooting gold, we'd rather hire a mage or a priest(ess)". But they've explained so much stuff by ceremonialists that sometimes I just rolled with it because it was a really cool idea.
Next game: Totally keeping it.
5. No hex clearing for the domain until average PC level is around at least level 6. That was actually from the first campaign that ended in the hex clearing phase, they've started at avg level 4 trying to secure a domain faster...and it was...ugh.
Campaign effect: Lots of deaths. Either boring or dangerous. Seriously, don't rush wilderness hex clearing.
Next game: Keeping it with a level 7 limit, because seriously, don't rush it.
6. Abstracting hex clearing for easy monsters ("it took one day to clear a goblin village, you got out with half of their treasure"). Some additional abstract rules for "easy" monsters with poison to see if someone dies due to a lucky roll. Another consequence of a first game.
Campaign effect: Less boring. You should only resolve the stuff where your players actually are in danger.
Next game: Keeping them. Still, I'd probably rewrite them to make them even less fiddly and probably taking more game time (which becomes more important than gold after a while).
7. Well, that's not just one rule, but several of them, making stuff less fiddly and easier to track. For example you get full amount of mercenaries at the start of a third week of the month if you start searching for them in the first week, because it was just easier to keep track of (or you could just collect them at the end of the month, whatever, just do it this month). Same with Call to Arms, IF it wasn't important (so if you're waiting for the imminent attack you'd prefer to get half of them in the first week, but otherwise it doesn't matter). You can sell your loot for 90% to the merchant's guild if you don't have Bargaining (or you could fiddle with mercantile ventures rules but please do it yourself). Stuff like that.
Campaign effect: Less tracking of unimportant stuff.
Next game: Keeping most of them.
8. BEST RULE. 12 month, each 28 days. 1st of every month is a new moon, 15th of every month is a full moon. That's it. THAT'S IT.
Campaign effect: Less tracking. Easier tracking of stuff that needs it. Seriously, to hell with all the calendars, it makes life soooo much easier with a simple rule like that ! Incidentally we started just calling months their usual names but given that most of them has a latin name anyway...it fits, so who cares. Also allows everything to become discrete, if it takes a week then it starts on monday and ends on sunday, again makes bookkeeping much easier. Really, it IS the best rule.
Next game: SOOOO KEEPING IT.
9. Remount rule from forums. Double overland movement if you have double amount of horses, x2.5 movement if x3 horses, x3 movement if x4 horses. If you need a warhorse for fighting, only one of them has to be, others can be simple riding horses but you might be caught riding one in case of a wilderness encounter.
Campaign effect: Players were running around with light cav+horse achers mercenaries, mostly with just x2 speed because they were hiring cavalry faster than buying horses (and leaving some of the mercs on the base to keep their speed). Not really a problem, and it was funny to figure out complications because of a huge herd running around.
Next game: Keeping it.
10. Availability of anything in the city depends on its demand multipliers. If mounts are at -1 multiplier, there are 1.5 times more of them, if they're are at -2 then you can get twice as much, and three times as much at -3. -33% / -50% / -66% for positive demand. You can still have the same availability as in the core rules but you pay either less or more with the same multipliers, use one or the other (this last rule is only for simple stuff needed "right now", can't use it for mercantile ventures).
Campaign effect: It was a nice way to make demand multipliers visible. Players were travelling to a specific city for their mounts (then sending henchmen there), and to another for mercenaries. Also shows Judge changes like "this town has famine because you failed to stop a Ravage ritual so food now costs more" or "there is a war nearby so less mercenaries are available", so they'd see it even without mercantile ventures. It conflicted with "less fiddly rules" but actually was pretty easy in play after a few times, when everyone figured out which city had what.
Next game: Keeping it. Using it more to show how the world around players changes.
11. Tracking time in dungeons by minutes instead of by turns, and removing "rest 1 turn out of 6" rule, by changing movement speed to match. It was mostly needed for me to roll for wandering monsters, and it was easier to say that "if your exploration speed is 120' then you can carefully explore one 10' square per minute" and so on. Still moved to 10-second rounds in combat, with 6 rounds per minute, which made combat length actually meaningful.
Campaign effect: Judge is less grumpy so monsters are less devious. Actually that might be because of other factors like nice 'zza, but I think this rule still helped.
Next game: Keeping it.
12. Shared worldbuilding, Dungeon World style. When I didn't have an immediate answer I just asked players what they think and got all kinds of interesting details which I was building upon myself later.
Campaign effect: Made my life easier, and also made world more interesting. The most visible effect was after one of the players, an avid anime fan, said that goblins must be like in "Goblin Slayer" (and forced us to watch it, it's like a campaign journal from a fucked up D&D world), which resulted in a burning hatred towards goblins, much more dangerous goblin lairs (because players expected it from me, actually made them very paranoid when goblin lairs were without ambushes) and a "goblinicide" campaign because players just started destroying everything with prejudice if there were goblinoids around. Diplomacy proficiency wasn't very important, but Goblin-Slaying was the most popular one.
Next game: Don't know. I plan to run a Borderlands game when "Secrets of Nethercity" comes out, so lots of details will already be there...but I think I'd at least leave some of the wordlbuilding to players, just a bit less, because it's really cool to learn something about the world that you didn't think about, players can really surprise you. Well, if I'd run it with my previous group then goblin hatred will stay, that's practically a given.
13. Player's rule: Shared money pool from which to buy stuff for the group, half of "Standard of Living" goes for the monthly allowance for small things, domain expenses are negotiated.
Campaign effect: Less bookkeeping by players, which is always good (domain and troop spreadsheets were large enough). Some domain stuff negotiations were...terse, but that was in character and was actually even cool.
Next game: Depends on the group.
14: Tables and rolls for EVERYTHING. If I don't have a table for that already, I'd just quickly create it and roll on it, because I usually don't want to know what would happen. Most of the time I prefer to be a referee and a guy listening to a shared story, not a storyteller. Say, if players go to the city mayor and ask him if he has a job for him, I don't know ! But I can just roll for it (last number on d3, d4, d6, d8, depends on what I feel right) then if he has something I just roll on a "what job (s)he has" table and figure out what actually would happen. It started as an experiment but I quickly realized that it spares me from bad modules and makes the game world a living breathing thing, if sometimes a bit fucked up in the head (probably bipolar).
Campaign effect: Complete unpredictability, within reason. For me as well. I had some specific story plots but they were mostly guidelines for me to have something unifying, but when it conflicted with a roll, I used a roll result because that's just more interesting.
Next game: Keeping it. Seriously, it's the best part of the game for me. My players don't know what happens next, and I want that sense of wonder and surprise as well, it keeps me engaged. JUST NEED MORE TABLES.
Eh. That's probably way too much already. Most of the rules came from actual play and were just changes or additions to our cheatsheets so it wasn't really hard to keep track of (we looked up all kinds of stuff in the cheatsheets anyway). Giving players more responsibilities helped too I guess. We had more rules, but mostly simple stuff like "what would happen if", which was mostly solved with a simple ruling or a quick search on forums for someone else's rules.
What are your thoughts ? Did you have something similar in your games ? What were your results ? Did you like it ? I'm pretty sure many of those rules are specific to the Judge and the players so it worked for us but probably won't work for everyone else, and it's interesting to hear about your experience.
Also share some insane stories about events your campaigns that happened because of small rule changes, haha.