Maybe the player can get the special ingredient from the Dizzy's facility with an event happening while you roam the halls and based on how many blobs you got at the facility or something else,you can get a blueberry from a scientist and you can decide to either eat it and slowly turn into a blueberry on the following days which is not permanent,the effect wears off after a few days or decide to bake some food,perhaps only one blueberry pie that you can eat and permaberry yourself and being on the weight stage of barely mobile after a few days,but you can still do things like running or some similar stuff,if you have a high fitness percentage,because you are more filled with more juice and some fat on you,you can still go to higher weight stages if you are a permaberry,but no lower than barely mobile or you can add a juicing machine that lets you go to lower weight stages.If you go with the blueberry thing,then the skin of the one that has eaten the berry can turn blue and the hair can be a shade of blue that you cannot get at the hairdresser and you can change the hair color to another color at the shop,also maybe lighter girls can have lighter shades of blue and the darker ones a darker shade of blue,you can darken the lighter shade of blue at a tanning salon or the beach,it can work like tanning.This whole thing can work on the girls,also i just put the player first,since they are the one getting the special ingredient.Also,maybe if there would be lactating in the future,the berried character can leak blueberry juice or some mysterious juice with the only effects being slight weight gain and temporary change of skin tone and hair to shades of blue.
Just a suggestion.