Just compiling a tiny amount of general information to help the speculation along, including a picture of the t-shirt in question.

Wikipedia link
TV Tropes link

Random factoids (mostly common knowledge):
  • The shirt is for the new "Mutant Mayhem" movie
  • TMNT was created back in the mid 80s by Kevin Eastman and Peter Laird
  • There are four of them
  • They are each named after Italian Renaissance artists
  • Common taglines/phrases include "Heroes in a Half-Shell", "The Green Dudes with Attitudes", and "COWABUNGA, DUDES!"
  • They like pizza and live in the sewers
  • Eastman and Laird's original company was called Mirage Studios

Should probably also consider the locale being the New World. Most new jobs have some relation to the new expac, even though some of those are haphazardly taped together (like reaper's relation to Garleans, red mage's relation to Ala Mhigo, machinists and Ishgard). It could also easily be completely unrelated at this point, like how dancer came an expansion before we had access to Thavnair, and gunbreaker is only related through post-story content of Bozja.