It's usually not so much the design as the idea that sells the shirt. People buy shirts for themselves that somehow relate to something they care about: their pet, education/avocation/occupation, sports, hobbies, etc. People buy shirts for other people as gifts based on their shallow understanding of what matters to them and their near total lack of understanding of the person's hobbies/work etc. Lots of this last category get sold; most stay forevermore in a drawer.
You might start with what you are interested in, passionate about, and come up with some original ideas on those topics.
