That rule exists already.
It's called "stubborn"
GW doesn't appear to add all that many special rules to units anymore apart from BRB special rules (frenzy, hatred, fly), and the occasional centerpiece with wacky complex rules ( * cough * mortis engine ), but regular unitts don't really seem to anymore. I think GW will keep it simple and just keep ironbreakers as they are, with a pointreduction maybe, or give them a point of extra armour or something and nothing else. More complex (and interesting) special rules like reversed impact hits, or having their armoursave be less modified by strength than usual (like -0 for str4, -1 for str5 etc.) seems to be out of the question if we look at the designtrend for the last 5 books. The most complex thing a rank&file unit has gotten that I can think of, is maneaters choosing generic BRB special rules.