As long as we are posting our wildest theories, here's mine.

In FFT, Holy Knights, Rune Knights, and Sword Saints can use an attack called "Northswain's Strike" (or "North-Star Bone Crushing Strike" in Japanese) which has a unique KO effect. Agrias Oaks and Thunder God Cid both use this ability in their FFXIV boss forms too. Here's a little trivia I read on the FF wiki about the ability:

The concept of Northswain's Strike's additional instant death effect refers to the Chinese deity of Northern Dipper associated with death whose role is similar to that of the Grim Reaper.
Therefore, the shirts hint that the next job will be Holy Knight/Rune Knight/Sword Saint.