Don't focus on bitcoin theme t-shirt only since the market is not quite high for that matters just create a good design in relation to bitcoins so if the other user could see that then provably they will avail your product if they love what they see. As well create some good stuff and other things which can pass to the peoples taste since for sure people will not always wear a bitcoin t-shirt everyday so better to have an option so if people will seek for another design you can present the other one..