It's pretty much impossible. I have the vague idea of maybe having three different presets for character appearance in the way of skin tone and hair color, because I might be able to just hue shift those things to a decent enough level to allow that, but trying to take into account the players penis, chest and ass sizes requires a ridiculous amount of work. The average CG set (that I commission) has between 3-6 images in it, so if I wanted those to take into account one possible ass size variant, that doubles it to 6-12 versions, which just quickly becomes very tedious for the artists to deal with. For my next game, I plan to make the transformation more linear, so that the artwork can actually show it.