Instead of using a predetermined body type system, how about implementing a randomized 3-size system? Bust, waist, and hip measurements would be generated randomly, and the text strings would dynamically change when specific stats are exceeded.
The body type system is mostly a style choice; I wanted girls to have labelled body shapes such as hourglass, pear & apple. There's also dialogue and clothing unique to particular body shapes.
But in terms of sizing, it does actually already work a bit like you describe. Each character has eight body part sizes: neck, arms, chest, waist, belly, hips, thighs & bum. The gain patterns determine the basic ratio of these, but there is also a random element, such that not every apple-shaped girl will gain in exactly the same way.
I'd love to have more control over how my character gains in character creation as part of a future update then.
I wanna make a lipedema character that TRULY only gains in the ass, thighs, and hips to the point of being completely disproportionate. (Think Squidward having eaten too many Krabby Patties, except really hot instead of grotesque looking).
I'd love that because if I understand correctly, it could be used to be build an absolute monstrosity of a woman. Like, a 600 pound, wide, tall, and padded with fat woman, but technically "thin" in a relative sense.
You could even let the player (during character creation) set the averages of each stat for the world to guide how the randomly generated girls come out on average, which would let me create a whole world of very fat mini-giantesses who weigh an average of 600 pounds each and look every pound of it, but are perfectly healthy at that weight and thus classed by the game as "thin".