Much sadness on this announcement, but completely understandable...
New fruit isn't possible in the way that you might think. You could make an item that looks like a banana, smells like a banana, and tastes like a banana, but when you plant it, produces an orange tree instead. There's too much going on "behind the scenes" to support something like that.
"Grass seeds" are impossible as well, there's no coding in the game to support such an item.