It's kind of fun to think about. I think first you'd have to come up with the logistics before you try to see if it's doable in the code.
One idea:
- Nomadic Civs can only build a Settler variant after founding their Capital City
- Once this Settler variant settles a new city, a Trade Route is sent from the Capital to the new city.
- Over the course of it's journey, the Trade Route slowly transfers Population from the Capital to the new City B. If this Trade Route is pillaged, it grants the pillagers a Settler unit.
- Once the Capital reaches 0 Pop, the Trade Route ends and City B the new Capital
This one is sorta fun, because it allows you to turtle population, then send out several nomadic settlers at once. Having visual migrants on the map is also kind of intriguing.