Some Yazidis and Christians have begun returning home. This began even before the Mosul offensive, when they liberated some areas around Sinjar.
The problem is, many of the areas they're returning to are quite devastated from the fighting. It's going to be a long-term challenge to reconstitute these communities so that they can function economically, so that more of the IDPs can return. Part of the long-term return issue has to deal with how significant the threat of an ISIS insurgency will become after the actual territory they control, in the traditional sense, is liberated.