This is kind of an 'out there' idea but I saw something like it on another game and thought it was cool and immersive. An ability for the characters to gain/lose weight depending on how often and what they eat. Certain foods could cause weight gain (meat and/or other rich foods?) and others cause loss (plants/berries?) There could even just be one particular food that causes gain and another that causes loss in case it would be too annoying to constantly manage. I think it'd be especially nice if it was gradual and dynamic instead of just changing from say, body type A to C in an instant, too.
