Draft animals/ animal husbandry
The ability to have draft animals would be such a welcome feature for medevial engineers.
We could have yolks for animals to pull our plows (and no longer ourselves). Horses would improve the speed at which players move about the world. Cows could give a new resource: milk, which could be used to make cheeses. Chickens could provide eggs as well as meat and fletching for arrows.
The possiblites are endless. Not to mention that it would greatly improve imersion.