I think it'd be neat to have varying levels of workshop quality, e.g. "shack", "shop", "factory", and so on. Unlike product qualities, these would not be generated randomly; rather, they would be explicitly built. Quality would determine:
* Physical size (poor shops are smaller)
* Amount of materials needed to construct (larger shops need more)
* Special inputs needed to construct (poor shops need only the revelant skill and raw materials; better ones would need input from e.g. glassmakers when bottles or tubes are needed (stills, alchemists), metalsmiths when metal tools are needed (carpenters, forges), and so on).
* Inherent storage space (better shops would take longer to be cluttered; additionally, some raw materials could be stored directly in the shop ahead of time as an abstracted stockpile)
* Happiness of working in the shop (skilled dwarves demand good workshops, while dabblers are happy pretty much anywhere)
* Efficiency of production (when relevant, better shops use fewer consumable inputs, e.g. fuel)
* Quality ceiling on goods produced (bad tools and cramped space will prevent dwarves from working to their full potential, but the inverse is true for the better shops if you have the skill)
That last one is the kicker, since otherwise you'd pretty much just make more of the same type of shop. The goal is that early on, you'd only have the resources to make the cruddy shops, and they'd serve you adequately, but as time goes on you'd want to create better shops for your skilled artisans.
The motivation for this change is mainly in that I think it's kinda odd that there's no bootstrapping of workshops in DF. All you need for a fully-functional magma forge is an anvil (which you must import) and some rock. I think it'd be neat, and give players more goals to aim for, to have more of a progression in shop creation. It also creates more relations between different shops, thereby giving dwarves more to do. Something like this:
* Metalwork shack
** Requires one stone or wood to produce
** Enables metalcrafting but not weapon/armor forging
** Enables creation of basic metal tools
** Not magma-capable
* Metalwork shop
** Requires anvil, three stone, metal tools, and a bellows (leathershop output)
** Enables all metalcraft and forging with high quality ceiling
** Magma-capable
* Metalwork factory
** Requires anvil, three stone, three metal bars, metal tools, bellows, mechanisms
** Enables all metalcraft and forging with no quality ceiling
** Magma-capable
The basic idea is that the first shop allows you to, with crude handmade tools and a lot of fuel, make the anvil and tools you need to start making actually effective equipment. Then, once you've managed to get things going well, you could make a later upgrade to the mega-shop to keep your artisan happy and churning out those high-quality goods.