I've just recently started playing around with large-scale agriculture for the first time in this game.
It would be neat if things like rainfall and sunshine actually affected how much the crops grow, and how fast. Right now, it seems to be determined right when the seeds are planted, using your Agriculture skill. However, in real life, crops are heavily impacted by weather. It would be cool if you didn't necessarily know how much barley or peas you were going to get right off the bat, and instead you had to wait to see how the weather helped or hindered your crop. Perhaps your Agriculture skill could help ameliorate the negative effects of bad weather, allowing your crops to survive flooding or flourish despite insufficient sunlight. I know this is a very complicated idea, but I think the person who modeled the ice forming & melting simulations for this game could probably handle it
This year in the game, I also had another interesting thing happen: we had a late frost in spring, preventing me from planting crops for over a week. Some crops were already in the ground, but they survived the frost fine. I think it would be interesting if late frosts could destroy your crops - moderated again, of course, by agriculture skill. It would be frustrating, but that's the life of the farmer!