Nice! I suggest maybe making some of the sprites in different poses than in the official sprites, as it could help you make original sprites. But so far, so good!
If you want to re-sprite something, I'd suggest re-spriting the unevolved grass rabbit. If you want to sprite something new, something like a small mammal or insect would be easy. I'd suggest starting off with easier sprites like those and then moving on to more difficult ones.