Well, I have some ideas:
1. Introduction of animal NPCs, which would require food on a regular basis, but would produce e.g milk as a result (you get the idea). - Quite easy to implement, but a bit wonky and unrealistic.
2. Kiln-based animals, which would be just charcoal kilns, but mobile and with different inputs and outputs. E.g: food + time -> milk (for a cow/sheep/goat). - Also easy to implement, but IDK how the game would react to moving animal-like kilns. (Moving kilns, what?!)
3. Proper animals with realistic mechanic (thirst, food and environment at the very least), whose growth rate depends on the satisfaction level of those needs. They could be slaughtered and butchered, but could also die of natural causes. It would look like this: Food + water + appropriate temperature -> growth + milk + wool (if a sheep or a goat, only milk in case of a cow). - Harder to implement. but the behaviour would be the most realistic out of all of the options.