The only thing I'm interested in seeing being able to grow is the stomach. Currently, it's only possible to resize it through the customisation menu.
Another idea I consider curious is splitting muscles into different groups, so they can change independently; though I'm not sure how exactly to approach designing that.
Making the growth function non-linear sounds like something to investigate too. Make the initial growth, needed to consistently beat enemies, affect the model just a little, so we don't look completely ludicrous at small levels.