Well, dwarves fitting square pegs into round holes usually results in square holes or round pegs, depending on which is stronger.
I'd imagine the barest minimum for a workshop is a 1x1 passable space - like a quern. With that, and some clever workarounds, it could be possible to make some pretty sick sh!t. For example, boards and planks. Make a 1x2 carpenter's table, and have him process logs into boards and planks (which would have to be weapons, most likely). Then have logs and planks in different amounts be processable into furniture, at another little shop. Then you can separately make blades and handles for swords, and forge them together (even though it'll hardly make a difference). Stone beds will, of course, become possible. These will, effectively, be custom workshops(in the "plan out a room and fill it with stuff" sense) - except you won't have that little required feature called task automation. As in, you will first have to make the blades and handles, and only then you can make swords.
In spite of this, would it be possible to have an "uncancellable" task at a workshop? A task that, instead of being cancelled when its materials are not available, is shafted to the back of the queue? I realise some of the implications of this, and that you can already do something like this with the job manager, but being able to put a task "on hold" until its required components are available, so that it will automatically resume then, would be very useful.
Also, will there be completely no way to get rid of the standard workshops? Would it at least be possible to remap the keys so that the custom workshops can be built instead?
Mini edit: maybe add a tag like [STANDARD_WORKSHOPS] to the entities to indicate that all normal workshops are available to them? Removing them will leave only the custom ones, with which the modder will have to replace the originals.
Also also, how exactly will the new 'shops be buildable? Will they be just stuck at the end of the existing list, or maybe we might have categories that we can stick them into?
Addendum: It would really make sense to add specific restrictions for workshops. For example, adding a new "Bonecraft shop" to Goblins would make sense, so that they can make instruments and crafts out of bones, but the same workshop won't make much sense for humans or dwarves, though they still use bonecarving for ammo and armors.