[Selmo]

I've always imagined that there were other small towns and villages like Bree scattered across Eriador. They're not mentioned because they're not important to the story. They would be populated by people of mixed Numenorian and Northmen descent, as would the bulk of the population of old Arnor (There were not enough refugees from Numenor to establish an entirely Numenorian kingdom. Only the ruling elite would maintain anything approaching a pure blood line.) Tolkien also hints at Hobbits living outside of The Shire and Bree.
In the long years of relative peace after the defeat of Angmar, I can't believe that the population didn't grow.

The re-establishment of the Kingdom of Arnor would consist of uniting the people already living there under a central government in the rebuilt capital city. The only significant population shift would be the movement of a relitively small number of Rangers and Gondorians to the new capital to form the new ruling class.