There's an iconic monster every month, not just once-- that's why there are two shirts per month. The second January design is a Dalek T-shirt based on "The Dalek Invasion of Earth." If the first Doctor shirt is anything to go by, there are no character images on the Doctor shirts, only fake logos, so they wouldn't need a twelfth Doctor costume or look, although what they would
use for such a design is an open question. The company's announcement on Facebook coyly says "...and who knows what may appear in December."