I'm not sure this idea will spoil the theme of this game.
I know a "garden" isn't a "farm", but I think having birds or fishes in the pond may be interesting.
I have this idea when I hear sound effect when I play this game. You know, I hear bird singing. It's quite relaxing actually. In real life, some animals join us in our garden, and they add "life" in the garden.
I still have no idea what birds/fishes will do in the garden. Maybe, they can provide some additional functions, or just give us more spirit?
Anyway, this is a really interesting and relaxing game. Thanks for sharing your creativity with us. Good Job! :)