I do like this game concept and do greatly enjoy the height growth. Not a fan of muscle, though, so I only buy the treadmills. But as someone pointed out earlier, employees getting tall can make some problems happen. My two employees and a customer got stuck inside the gym and softlocked the game. Maybe perhaps, until a proper fix is added, you add a button somewhere that resets the NPC locations when it is pushed. Maybe behind the gym or something?
Another machine idea you could potentially add is a rowing machine for thigh/butt growth and a pectoral machine for chest growth. Would love to see those bosoms get bigger!