Unification will be an interesting part of the game, especially those that never happened historically, but could've realistically. Here is how to accomplish them.
The forming country must share at least one Primary Culture with the Unification country. For example, any country with either South German or North German Primary Culture can form Germany, which has both.
The forming country must have a lower country tier than the Unification country. Country tier is an inherent ‘status’ assigned to each country based on how it was historically perceived and gives Prestige based on how high the tier is. For example, a country that was historically regarded as a Kingdom gets a bit more Prestige than one that was a mere County.
This is a fairly weak effect, as the idea that some countries/titles are simply more inherently prestigious than others was falling out of fashion in the Victorian era, but serves as a useful way to determine which countries would, realistically, strive to change their identities, as it’s quite unlikely that Poland would want to turn itself into just Galicia-Volhynia, while Krakow might jump at that opportunity.
There is one exception to this rule. That being that a country may form a Major Unification of the same tier, if they are themselves not already a country that counts as a Major Unification. For example, the Empire of Austria can become the Empire of Germany despite not increasing their tier in the process.
The forming country must own or be the suzerain of the owner of a sufficient number of the integral State Regions of the Unification Country. Usually, what a Unification country considers to be its integral State Regions maps directly to the Cultural Homelands of its Primary Cultures, so the integral State Regions of Italy is any State Region that is considered to be the Cultural Homeland of either North Italian or South Italian culture.
Certain Unifications may require you to research technologies such as Nationalism or Pan-Nationalism to be available.