Sometimes simple is best. This shirt design by SeanSeen on Redbubble is a great example.
With all the vectors and digital processing you see on most shirts, it’s nice to see a nice, simple drawing with a really friendly style.
Take a look through the rest of his shirts, and you’ll see the patterns emerge like the yellow filled-in text, and the cute line drawings with minimal colour. These doodles really work well on shirts.