I wanted to take a pre-built child theme and see how simple it would be to customize it, since I normally customize the base "sample" child them. I've customized eleven40, since it's the newest and has responsive design. Even adding borders, header backgrounds etc was infinitely simpler with the new color styes - the hardest part was remembering to add those changes to the colors in the CSS.
I'd be interested to hear any feedback on where I took it. I called it "Numbers" ...I'm guessing you'll know why! :)