With players accounts being maxed people constantly running m10s and the fresh start servers coming out, people are craving the next challenge.
After recent updates the game has become self sufficient now in terms of being able to max every skill, find every item needed and do everything in the game you can now do naturally from in game drops.
If you add an iron man option you basically lock the trading post and trading from the players character on character creation and give the player a small indicator next to their name, or a title which basically says iron man or give it it’s own name.