Easiest way to expend would be "Futa" option gives something new to grow. Nipple expention. optional to fadeing, how about orgasm (pink rooms)?
would be nice to have some secrets/traps/puzzels (besides the button that need 5500+) where you need to have a min/max amount of silicone. lose board she falls town if you have above X, lose by stuck in the wall. wish for a bathtub scene where she bath in the silicone and slowly grows.
Since the Dev probably want the game to be beatable without upgrades, place them in optional path.