An option is to build the case entirely of colored squares with a flower on a complementary or contrasting color on top. This is an example of different types of flowers and on the right is a single square.
The squares could be of different type of yarns which would give variety to their size and textures
Mural instead of i phone case: Another option is to play with scraps of fabric as the main focal point and add embroidery, crochet and doodles as accents through out.