I’ve talked about this a bit in my Northern economic development plan, but the foundation of any Northern immigration plan is going to have to be something akin to the German princes offering vacant land for free (as well as tax exemptions and other privileges) to anyone willing to settle in their lands after the Thirty Years War had depopulated entire regions.
We’re already starting to see that a little bit with the 100,000 willdings who just arrived, growing the North’s population by 2.5%, getting settled on the Gift. But, given its extremely low population density, the North is full of Gifts yet ungifted.
In terms of where these people would come from, you’re going to want people who are already familiar with agriculture and livestock herding, so I would concentrate on the younger sons of landholding yeomen who aren’t going to inherit the family farm, as well as any and all peasants who don’t hold land (your crofters, your cotters, your sharecroppers, etc.). In so far as the North is also looking to industrialize, I would target journeymen and apprentices from the textiles industries of Myr, Tyrosh, Norvos, etc.