Have you ever considered maybe doing something with April from the 2012 TMNT show? While she's technically a teen, I would think her younger, more petite body could work with your style. As for what she'd be doing, maybe she's squatting in an area of the sewers and unloading.
Either way, it'll be fun to see what you've got in store next. I'm certainly eager to see the completed version of that Gwen animation.