Whenever they belong to the same faction, they should honestly just rework the character creation screen and make most allied races as a sub-race template within its core race.
Seriously, why would we need the Dark Irons to be listed separately at all?
If their specific name and traits came tied within the choice of a Dark Iron template from the customization menu, they can be just listed under your regular Dwarf on the main character creation page.
Less clutter on that one page, more options within the customization section, and the fans of specific subraces being added with the customization options won’t feel treated any worse than those who got the Allied Races.
Win win for Blizzard and the players with a minimum amount of work, as with the introduction of Exile’s Reach this would not require any new starting zones and racial quest areas.