Design as a field seems to be exploding. Over the last 2 decades, new areas like UI design, human interaction design and game design have come up. Some areas that I find related have also grown, such as information visualization. Even more far flung fields like business model design or choice architecture are becoming prominent.
The common theme of good design - good to individuals and useful to society - has the common theme of "making it easier". This is really from a cognitive sense. Making functions obvious, reducing errors, making choices simpler, or better all end up using fewer cognitive resources and therefore easier for the user. In a way, its cognitive judo. How about a Design firm called Judo, with a tag line - Making It Easier !