Hey I'm not sure if anyone noticed this, but on the description page for the "FWC" T-Shirt the description reads:
Have you seen us wearing shirts like these? Drape yourself in an enigma from our future release.
Note the word future in the description. In other descriptions, like the ones for New Monarchy and Dead Orbit, clues are given to the name of the of the faction.
Dead Orbit:
You can be a dead ringer for a Bungie employee. Decorate your chest with this mysterious crest and watch the questions fly.
New Monarchy:
For Bungie Day, we are giving you the shirts off our backs. Adorn yourself with a mysterious icon fit for a king .
Notice the words dead (for Dead Orbit) and king (for New Monarchy). These descriptions lead me to believe the word "future" definitely stands for the "F" in FWC.