Some countries could be considered 'settler societies' which would have reduced costs for creating colonial societies. Brazil, Canada, USA, Australia, Argentina, New Zealand, Cuba. But just because someone creates a colonial society *doesn't* mean that anyone will necessarily come. Afghanistan creating a colonial society encouraging people to move there would probably not be very successful. Some of the biggest things to consider in creating colonial societies would be..
1. How much money is given to the colonial society: This would be like a slider, you can raise or lower the amount of money given to a colonial society like anything else in your budget. Colonial societies could be very expensive. The cost would represent things like paying the passage of new people coming to your country, some basic education (literacy), the health costs of screening new people for disease and vaccinating them, giving them free land, farm tools, and all of the other myriad things that countries did for immigrants in the 19th and early 20th century as they encouraged people to move there. A successful colonial society might be very expensive, but it might be very successful in encouraging people to move to your country. For settler societies, a modifier might make maintaining colonial societies much cheaper.
2. How open the society is to immigrants in general: In the 19th century Russia was hellish for Jews, but other groups like Germans migrated there quite freely, especially religious groups like Lutherans and Mennonites. Crimean, Caucasian and Volga Germans moved to the Russian Empire in large numbers because they were given generous subsidies and encouraged to improve the land. When Russia took a turn for the worse in the mid-late 19th century, some of these Germans left for America. By encouraging one group of people to come to Russia, they were decreasing the power of other groups (the native Tatars and other ethnicities whose lands had recently been conquered) and hence increasing the relative power of the Russians who also moved to these places. A state with 300,000 Tatars and 200,000 Russians has the Russians at a disadvantage, but one with 150,000 Germans, 150,000 Tatars and 200,000 Russians has the Russians in the majority. But in general, a society with high consciousness, liberal reforms and generous social welfare should be seen as more encouraging of immigrants, but even a country like Russia could set up a colonial society for Germans in Crimea and get a substantial number of immigrants so long as the subsidies were right, while at the same time driving out Jews through antagonistic social policy.
One interesting thing is something like this...
In general before 1860 Northern Democrats promoted easy land ownership and Whigs and Southern Democrats resisted. The Southerners resisted Homestead Acts because it supported the growth of a free farmer population that might oppose slavery.
When the Republican party came to power in 1860 they promoted a free land policy — notably the Homestead Act of 1862, coupled with railroad land grants that opened cheap (but not free) lands for settlers.
Thus a political party with slavery as one of their platforms might have a limit on how much money you can devote to a colonial society, even in a settler country like the USA, a hard limit on what percentage of the slider you can fund like the hard limit on taxes in a socialist or anarcho-capitalist government in Vicky 1. There are many ways you could model different governments and how they could fund colonial societies. If a party based on a single ethnicity came to power in a nation, they might disband any settler society not based on their ethnicity for instance, or basically refuse funding (For instance if a Boer party took power in South Africa, they might think why should a Boer government in South Africa pay money to encourage more English people to come there?)
I think this would make the game a lot more interesting and turn immigration from something that is in Vicky 1 basically a completely random event that at best results in a bunch of people moving to your country and completely forgetting everything they used to be in the course of a year to become normal national POPs into something that is more of a tool, much as countries used the movement of population in the 19th and 20th centuries, to gain greater control of marginal and newly conquered areas, to promote their national culture, to fight their enemies and so on and so forth. It would also allow the creation of settler societies whose countries were mostly populated by Europeans during this era, the Australians, the New Zealanders, the Argentines and Brazilians, the Dominicans, as well as some groups which never fully took hold in our timeline but which might well have if things had gone differently, the Pied-Noirs in Algeria, the Italian colonists in Libya (at one time more than 10% of the population there), the Jews in the Ottoman Empire (later to be much more successful lobbying the British).
It also opens the way for the odd occurrence that makes a sandbox type game fun, China decides that the high militancy of the Manchu people makes them undesirable and encourages them to emigrate, they form a free colonial society which approaches Mexico seeking to move to the sparse California wilderness. Due to California being much more heavily populated during this time, the USA never gains control though 50 years later you have a civil war in Mexico and eventually a Manchu-populated free nation of California is the result! It's definitely a counterfactual but it's also something that's not much stranger than a community of European Jews managing to move millions of people to what would become Israel over a century, completely changing the language and character of the area, nor if you think about it, that much more strange than a bunch of Europeans moving to California themselves.