What about adding an actual immigration policy? Right now, all I can see that affects the level of immigration is boarder controls. In Canada, we have a targeted number of yearly immigrants, such as 300,000. It is also based on level of education, wealth, and other factors for individual immigrants. The gov’t tweaks this yearly depending on job demand and other factors. This would be a great addition. You could choose how many and how easy it is for people to immigrant to your country. There’s also the issue of family members coming and being sponsored. There is also the problem of people coming and getting citizenship but not really living here, but they get access to health care and other services.
I think immigration is a bit simple in this game.