I'm considering a mod, inspired by someone else's attempt. You can sheer creatures now, for wool, but you can sheer any tissue off them. This would allow, for example, for the Boulder Crab, a large cavern creature that harvests rocks for chemical energy and to grow its shell. The shell can be sheered, producing a number of useless garbage tissues. However, these would be automatically taken to the forge (or elsewhere) and turned into their finished product. A marble crab would allow you to harvest marble tissue, which is then put through the workshop and produces marble boulders. Or copper. Or emeralds. In this way, you could subsist your metal production on Boulder Crabs, albeit at a rate of ~1 boulder per season, per crab, with some rarer harvests. Like, a precious metal crab could produce a silver and gold shell that would produce 1 silver per season, 1 gold every two seasons, and 1 platinum every 2 years. Gem crabs may produce a lot of "junk" gems, but have a slower production of valuable gems. They wouldn't be bad as war creatures either, since they have hides of stone or metal.
I have NOT worked on this yet, but if/when I do, they'll be very expensive, slow-breeding [COMMON_DOMESTIC] animals, and the reactions that turn the tissue into usable material would be automatic (the way that "Process Fat" is automatic) so that you could sheer them and leave them alone. A similar style of enemy could be devised, which when butchered, the resulting tissue could be taken to the workshop and turned into materials, which would allow people to play on resource-devoid maps and still have nice things.
Sorry, got carried away and ranted a bit