How about make Upgrades for the greenHouse, similar to the house, "go to Robin and Purchase a Up to GreenHouse after you get it with the Joja or Community Center"?
The greenhouse is a small addition to the farm that allows you to farm little bits no matter the season. Surely expanding it would be great in the sense of earning money, but I'm convinced it's not necessary. During winter it's a nice addition, you need to pick the crops to plant in the greenhouse carefully to earn the most from the little space you got. During the other seasons the rest of your farm should be plenty of land to focus and grow crops on.
