"Study your successes, not your failures. If you study your successes, you learn how to succeed. If your study your failures, you learn how to fail." - Richard Hamming, You and Your Research.
I'm not actually proposing to run another ACKS campaign. This is more like a thought-experiment derived from the previous (relative) success of the Bjornaborg campaign, after which my players remarked positively on their relative ownership of the world and their dwarven engineers.
Part of the issue with ACKS is that I never want to map a dungeon ever again so long as I live. So the question - can I get players to do it for me? The answer, I suspect, is yes. Previously I've had several players take great pleasure in designing and mapping their castles in ACKS. "Reverse dungeon" is already a thing, with the players running defense. This would look a lot like that, but with some base-building aspects on top of it. Coming at it from the ACKS side, it's extremely granular but benefits from single-domain-for-entire-party, which should be good for cohesion (if you can get buy-in on the idea at all). This is like, domains for third-level characters.
You get a population of dwarves. Cut it up into pools of anonymous Laborers, Craftsdwarves, and Soldiery, with a small number of named NPCs (eg, veterans / heroes, master craftsdwarves, sages). Each laborer can produce 10gp/mo of food (provided space to grow it), construction, or mining. Craftsdwarves can supervise construction as engineers, produce goods for export or use, or build traps and siege weapons at about 20gp/mo. Soldiers, of course, train, patrol, and fight (or labor).
Let's do some math. We're going to assume Absolutist Survivalist Autarchy Juche Dworth Korea economics with no useless children as our baseline, from each according to their ability, to each according to the whims of the player characters. I'm also going to take a few gross liberties with ACKS' math.
Farming - one laborer produces 10gp/mo (9 really, per the Labor proficiency, but 10 for ease of use). Per Campaigns, supply costs for infantry are 0.5gp/wk/individual, so one farmer supports himself plus four others. Great. The really interesting question is "how much land do you need to do that?" This is something that Dwarf Fortress is really bad about; farmland is rarely a problem except in the early game in certain embarks (glacier, salt water, &c), because productivity per unit of farmland is very high. I'm sure ACKS has the answer to this question, and that answer is almost certainly "even with mushroom crops, there's no way in hell you're going to manually dig a farm large enough to support anybody". So you're probably left with surface farming or cavern farming, and those are both areas you're going to need to secure by force and fortification. Looking at the Axioms Peasant Economics issue (goddamnit) they have 30 acres of land producing ~160gp/year of food (but with more labor), so ~5gp/acre/year, so if you're producing 10gp/mo you're looking at like 22 acres. An acre is ~44000 square feet, or a 210' x 210' area. So... yeah, if you want to farm underground, you're going to need to do a lot of digging. You could probably get into some decent surface-world Civ-type gaming if you had the right hexmap scale; a half-mile hex contains 24 farms, a quarter-mile hex contains 6, so somewhere in that range is probably about right. A nice feature of operating at this scale is that rapid response is possible; if goblins are burning the outlying farms a mile from the fortress, it takes news 10 minutes to arrive at a run, a few minutes to reach the PCs in the fortress, and then 10 minutes at a run to get to the action... which means some peasants might still be alive. That is, if you have roads and bridges, of course. Maybe faster if you have lookout towers and soldiers on duty; because hey, you can see smoke from a long way away! So the PC choice structure / play loop here looks like "choose which hexes to farm, where to build logistics and fortifications so we can defend them, defending them when they get attacked, tracking the attackers back to their lair, and clearing it." Depending on how complicated you want to get, you could also get into like, building huts for your peasants and clearing woods for raw materials. Might also make sense to have no surface farm production during winter, so you want to shift mining labor to farming in the summer / fall, stock up food, and then shift farming labor to mining in the winter.
Right, the other option is farming caverns. I'm going to assume comparable land productivity between surface and caverns for ease of us (actually a terrible assumption, because power per square foot on the surface is way higher due to sunlight, which is why there isn't a whole lot of biomass density in caverns IRL). Unlike the surface, caverns are mostly closed, so instead of a hex map it might make sense to just use a graph of connected nodes, each of which has some size in arable land, and each edge of which has some distance for travel purposes. In the spirit of jayquaying, throw in a smattering of surface-cavern links; very reasonable places to build fortifications. Could also very easily do multiple layers / depths of caverns, underground rivers, &c. PCs explore caverns, locate monster lairs and desirable special features, choose which caverns to clear and secure via fortifications, and play reactive defense.
Mining - Mostly we're interested in mining for space, although mining for raw resources is also worth thinking about. ACKS has somewhat conflicting numbers for mining for space; in Core, a 10x10x10 section of dungeon hallway costs 500gp, which is 50 dwarf-months of labor. Ouch. In DaW: Campaigns, though, the cost of siege-mining is 1gp per 20 cubic feet, or 50gp for the same volume, a mere 5 dwarf-months. I guess the important difference here is the flagstone floor you get in the dungeon hallway. Let's go with the 50gp/cube estimate. So if you have, say, 25 laborers, you can allocate 5 to farming a quarter-mile hex (producing 50gp/mo of food, which supports 25 dwarves) and the other 20 to mining, and dig four 10x10x10 cubes in your fortress per month. If you happen to have an ore vein, this can also produce raw materials of value (would probably steal a note from How to Host a Dungeon here, and figure out ore vein locations by dropping dice on graph paper). Given that it takes about 5 dwarves to dig a cube, and about 5 dwarves to farm a quarter-mile hex, five dwarves producing 50gp/mo might just be the right size to track labor in.
Labor is probably going to be a limiting factor of construction, generally. Given the presence of surface-cavern links, it might make sense to secure that initial cavern and fortify all the links out, even if those fortifications are just wooden palisades, because then you don't need to dig out living space (huts-in-a-cavern are cheaper, but less defensible, than excavation).
Craftsdwarves - assuming a mix of apprentices and journeymen, I'm willing to call it 20gp/mo of production per craftsdwarf for ease of use. That production can be finished goods for export (cut gems, enameled beer steins, whatever), weapons and armor, siege weapons and ammunition, traps (per Player's Companion... though traps are crazy-expensive), or "Specialist" support for military, which is a cost of about 1gp/soldier/month. Craftsdwarves are scarcer than laborers and more productive, so you probably want to house them in or near the fortress. They may or may not need workshops and tools, depending on the detail you want. Again, very nice to manage in groups of 5, for 100gp/mo production.
TODO seasonal or annual check to see if a craftsdwarf turns into a named master craftsdwarf. Might involve Fell Mood, PC quest for unusual materials. Not that it would be worth the hassle, because master craftsdwarves don't improve production that much... unless you make them grandmasters from the Heroic book and let them start making masterwork / semi-magic items. That it might be worth worrying about.
Soldiers - this is kinda the easy one, since there's so much material already in Domains at War about them. Comes in two flavors, heavy infantry and crossbow (and maybe furies). Need both food / supply from labor and specialist upkeep from craftsdwarves. Station them in fortifications or watchtowers, send them on a regular patrol route through certain hexes, keep them close at hand so they can support PC actions, whatever. Probably can't have more than a third of the militia or so on watch / available at the drop of a hat, but should be able to bring the whole force to bear with a day's notice. Nice to group into 6-dwarf squads (for fighting in 10x10 squares) or 30-dwarf platoons. Combat against beastmen (goblins and orcs) is probably going to be on warband / platoon or even squad scale.
TODO occasional check to see if a soldier turns into a named / classed hero, subsequently available for henching; might only happen through combat. Heroes can train laborers to be soldiers, per Campaigns (and then you use craftsdwarf labor to equip them).
TODO priests? Sacrificing food and goods for blessings. Prevent disease and undead? Praise be unto Armok!
TODO morale, consuming excess production for a morale bonus. In the case of excess food, this is a festival; in case of excess crafts, it's gift-giving and private property.
Caravans - every season except winter, a caravan arrives and you can trade them stuff; useless tchotchkes, excess food, goblin prisoners, smoked chimera meat, whatever, and purchase weapons or food, or hire labor from them, in return. Since we don't pay anyone wages, arguably the "fair" / ACKSonomically sound thing to do would be to convert excess production into migrants at slave-labor rates, of 33 times their monthly wage. Since we have to produce the goods we're using to pay that price, we can take three months of production and use that to pay for a migrant, so for every 11 dwarves allocated just to exports for an entire season we could get a new dwarf. That's kinda harsh; I expect casualties would outpace population growth. Another option is that you export some goods and then the amount you export earns you a certain number of rolls on the Migrant Table... as well as on the Monsters Table, with results including goblins and dragons, of course. Fame for production is a double-edged sword.
So that's all pretty straightforward. The real remaining trouble is PCs. What do they do? How do they earn XP? Why do they care about this hole in the ground full of dwarves? We've broken the core adventuring heist "get the money and get out" gameplay loop of ACKS by making the game largely defensive. Could award XP for fortress / domain production, consumption, or exportation, plus cash earned from fighting and killing monsters I guess. In terms of class mix, Vaultguard, Fury, Craftpriest, and Delver are all very reasonable classes. A dwarf surface-explorer counterpart to the Delver's underground exploration might be reasonable. A dwarf ceremonialist from the Heroic book might be reasonable too, to get some effects that you can't with divine magic. Human bards, mercenaries, scholars, and monster hunters are not unusual long-term guests, and there's even precedent for elves leading dwarf fortresses. So maybe party composition is not that big a deal.
In terms of adventuring activities, there's actually a mix of reactive and proactive possibilities. Reactive adventures include stopping monsters from killing all your peasants and seeking out rare materials for fell moods. Proactive adventures include cavern exploration, treasure maps?, and clearing out monster lairs that have raided you previously. Probably a reasonable place for clocks - if you defeat a raid, it's only a matter of time until there's a stronger follow-up. Likewise, over time Monster Table results build up, and those lairs may merge. So a proactive policy of taking care of threats before they get worse is probably wise...
Oh yeah, and the endgame - fortress falls to darkness, and a new party gets to explore / loot it.