After my post on fertilizer.... and then looking into how farming's not changed too much since haven. I have had yet another (fanfare!) idea. I have a feeling this one has been done before or at least in spirit, so sorry for the original idea person.
How about a fancy plow!
Well, it's a harvester. It's basically a plow that harvests crops and replants seeds. It has three storage parts, one for the produce, one for the seeds you want to plant (yes you need to have the seeds to begin with) and one for the seeds you are getting. (it will also plow the land). maybe its storage "bins" could be removable, like other vehicles such as the boat, cart or wheelbarrow.
To make one, you would need the same stuff for a metal plow, plus gears (2 scythes, as they are cool!) and some materials for the two leather bags or whatever for the additional storage. it would use cows or horses to increase the "tech" requirements and stop it from being used from the outset. it would need a specific skill for it to be used, and it should not be cheap.