I've talked before about doing more "base-building" style gameplay in the mid-levels. I think that ACKS already has pretty much all the numbers that you'd need to do this; there might be a little extrapolation but I think most of the work is just putting it all together. Much as the domain rules are very general and bore concretization into simple domains.
Players will probably want workshops or blacksmiths or something like that in their hall/compound, and the numbers are easy so let's start there. A master craftsman plus two journeymen and four apprentices produce a total of 130gp/mo, with total wages of 100gp/mo. If you paid them up front at the 30-to-one typical ratio, that's 3kgp in wages to hire them indefinitely. They also need a stone building, call it equivalent to a townhouse at 1200gp, and a Craftsman's Workshop worth of tools per Domains at War: Campaigns, for 350gp (although we're only using 7/8 of it, so maybe call it 300gp in the interest of round numbers). So that gives us a grand total of 4500gp up-front cost for 130gp/mo of production. Neither of these numbers are especially convenient. If we wanted nice round numbers, we could probably go up to 5000gp up front for 150gp/mo of production in a 30x30 footprint.
And we can use that for pretty much any skilled craftsman producing small-scale goods (ie, not ships or siege weapons or other big construction projects that use a lot of unskilled labor, and not things that need lots of land like healing herb gardens or breeding horses). Blacksmith? Obviously. Armorer? Sure. Brewery? No problem. Military oil refinery? Why not (150gp/mo of military oil is kind of a lot; 75 flasks a month is more than I've ever seen a party go through. But maybe they would if they had it!). Gunpowder grinder? What could go wrong?
I like that 5kgp is totally within the reach of early-wilderness-level adventurers. This is a solution for the problem of "we have money but we can't buy the things we want in this market because it's too small". 5kgp for 150/mo is also comparable to the return on agricultural investments (but since it's in concrete goods, it's much less flexible); it will be more profitable in areas with low land value or high garrison costs, but worse in areas with good land value and low garrison costs. So that's sort of interesting. If you wanted to get real detailed with it, you could also have each workshop add ~3 urban families to the domain it's built in (a master craftsman is eminently marriageable and likely to start a family when he arrives, while the journeymen and apprentices aren't but there are a bunch of them).
And then the limiting factor on building workshops is mostly finding master craftsmen and inducing them into your service. I think making them available as Alchemists or Chirurgeons is correct, since those both also have 3 levels of a General proficiency.
We may also want some chance of disaster, like from the Investment rules in Axioms. I expect players might accumulate a lot of workshops and other comparable facilities, so I don't really want a per-facility roll. I would probably need to think more about what the right way to do this is, or if we want it at all.