Fun game! If you ever make update I would love to play them too!
It would be cool if you can introduce some more difficult levels. What could be an idea is to introduce bonus objectives to allow the player to sort of decide their own difficulty level. For example , you could introduce a special kind of sheep (or other animal) that does not need to be fenced in in order to complete the level, but if you can manage to do it anyway your bonus counter goes up!