In this image, we see exponential difficulty. At best, this works in some arcade game, but overall even in those, it is just frustrating.
However, imagine a ninja warrior game, where failure to complete the course in time, results in 5-10 extra kilo. You gotta get through it within some number of minutes, and each time you make it, it doubles in difficulty.
In the same vein, here is a different kinda fetish game: Inch by Inch by Dare Looks, LevelXProject
You have to make an antidote before you become too small to hold the vials and ingredients. This is combined with a full customization suite of shrinking speed, min and max size, the frequency of your shrinking and a cheat mode if you feel like it.
You can make it too difficult to make any kind of real progress with about 8 steps, each taking minutes to complete at 3 workstations. While you have 2 minutes before you are too small.
This thread is about other game mechanics that absolutely suck the fun out of most games, but are almost perfect for fetish games. Transformation or other.