You feel adding an optional extra mode

If it doesn’t have a separate progression system, it’s not really optional—not in the way that you imply. It changes the incentive structures of the game as a whole, which means the whole game must be played differently.

As a basic excercise, let’s think about a game with a dark hallway that has scary monsters. Most players don’t want to walk down a dark hallway with scary monsters, so how do you get them to?