The subject of the classical four humors came up in my 5th grade class today. According to Galen, there were four fluids that controlled the personality: Black Bile, Yellow Bile, Phlegm, and Blood. The corresponding personality types were Melancholic, Choleric, Phlegmatic, and Sanguine, meaning “Sad and Thoughtful,” “Intense and Angry,” “Relaxed and Easy-Going” and “Outgoing and Sociable,” roughly speaking.
It might be interesting to use these as character stats, but translated into RPG terms. For example, Investigation, Aggression, Stealth, and Social. Presuming that the values for these stats would be distributed differently among the players, you’d have characters that would have to focus on very different play styles in order to succeed.