An easy to follow guide of how they work would be helpful. Mixing grain can be quite complex until you get used to the quantities you need to use.

More uses for the available grains would be great. Perhaps feeding specific grain could benefit the horses training or something.

As someone commented above, feeding health grain instead of hay is very expensive, perhaps this could be adjusted somehow or it could have a different effect than feeding hay.

It would be great if the crop fields could have other uses. As mentioned growing carrots and apples for the horses.
Or maybe growing other fruits and vegetables which could be given to our riders for an extra boost in training or performance.
Straw for bedding in the barns perhaps? Bedding would involve a whole new aspect to the game though.

It would be awesome if we could gift or privately sell hay to another stable.