So yea, believe it or not, I'm not the first guy to think of making a story about shapes and their imagined culture! Not only are there a good number of shape-based comics out there already, but there is also Edwin A. Abbott's "Flatland,"
written in the historic year of 1884. Not only is is a story about shapes and their social hierarchy, it's also a famous piece of satire about Victorian culture, and you know how I love my satire!
My recommendation? If you like Circle Versus Square, read Flatland: A Romance Of Many Dimensions.