For some time now, I have planned to do a scene which involves gradual transformation of a female character. The concept is that the female character is pregnant of a therianthrope(a were creature). Through blood transmission between the child(ren) and the mother, the mother would undergo resisted transformation in order to carry and birth her litter. However, this is unprecedented in the setting of the characters and therefore, the results are uncertain. In addition, from a purely erotic standpoint, the mother can be made pregnant with more children.
For this, I would like to design a game system that allows the player to decide what transformation she may undergo, what transformation she must undergo and tracking the progress and effects of the pregnancy and eventual birth. The more pregnant the character becomes, the more quickly or dramatic she transforms. If the player wishes to give the character multiple breasts, then the character must first increase the amount of babies she can carry. If the player wishes to gain strength, then she must gain fur first. Etc.
How would I go about tracking all these things...?