Gardening can already be done in a billion places, and I can see why the devs don't want to increase it's size; size would just make it the meta mid/max farming lot.
Underground farming would be neat, because honestly why not? but I don't think the greenhouse would be ideal for that, it would look a little silly.
I think they should look into reworking greenhouses into extendable tent like structure and players can just see in the overworld and walk under (much like they're trying to do with current houses) and anything inside them is uneffects by snow~ it would just basically be an expensive mound bed.