What do we think about sub communities?

Right now, communities live on a single flat level. But what if we were able to have sub-communities? Communities could be grouped under larger “parent” topics. Each parent could aggregate all posts from its children, giving people both a focused and big-picture view. For example: Food could be a parent, with Baking, Vegan Cooking, FoodPrep, etc. Obviously we don’t want digg to become the good old Yahoo directory with 20 layers of children (IYKYK), but maybe certain topics could have parents?