Next Villain Should Be: Female
As most movies have shown over the years - whether live-action or animated - there would usually be a villain of sorts that would stop at nothing to succeed in getting what they want and stopping/eliminating those who stand in their way.
Now, for the next Kung Fu Panda (#3), the next villain should be female. Why? Because there have been many villains that were cunning, ruthless, or even manipulative, but they were mostly guys, for it being a woman, that would change everything in the 3rd film (mainly for Po).
Does Po even have the nerve to strike a woman? And since the setting is in China, there has been some beliefs (in real-life) that a man shouldn't hit a woman and the Chinese culture looks toward that.
As for what type of animal? I'm leaving that open to anybody.