Hello,
I think that the inclusion of an iron man type difficulty would be great for the next fall out series. By iron man difficulty what I'm refering to is the auto saving that would occur and the avatar/players choices are permenent. There is no going back an reloading. (For an example, see XCOM: Enemy Unknown; although obviously the implementation would be more difficult in an RPG). This would make the game playthough much more difficult and in my opinion much more enjoyable. I think that the ability to not (especially on a PC) reload and quick save at any point in the game means that the player must be careful about the choices they make.
As clarification, I'm not saying this should replace hardcoe mode (which I do enjoy), but rather be an alternative setting that is possible. I also feel that the inclusion of such a feature some things as game difficulty settings and such should be locked and unable to lower difficulties.
A single playthrough (or up to a certain game checkpoint) before the user is able to select iron man mode I feel would be nice. I think iron man mode is best played after the user has played through the game once and is looking for a challenge that isn't done with enemy HP and dmg increases.