more cultures
I like the idea however I have some reservations that I am not sure a series of "events" can resolve. I also do not want to see it make the game harder to replicate historical reality..i.e. the one province before annexation. (The Ottoman Empire fought one war to conquer all of the Memluke, Hedjze and Serbian kingdoms..not 2 one to take their territories and anothr to take their capital.)
1) How do you deal with forceable assimilations? There is a historical trend for cultures to be absorbed and eliminated. examples are the indians on the Eastern Seabord of the USA, the variety of indiginous populations in Mexico, the Armenians, Greeks, and Latins throughout Anatolia and the Levant. In 1419 France was a multitude of similar yet different cultures and dialects, but the centralization process lead to the death of most of those cultures and dialects until you came to a modern France by 1700..how is this to be replicated???
I see three options:
1) How do you deal with countries that are one ethncity? Eliminating a countries ethnicity...i.e. France not being french, but anyhting would serve as a simple solution to the above? In this way you can take over a polygot of cultures and not have the ethnicity problem. The example may be better suited to super assimilation empires i.e. Byzantines, Ottomans, Spain, China, Persia, England...where the subject groups adopeted the language and perdominate aspects of the conqueror.
2) An event that allows you to culturally convert the conquered provinces with negative stability, money and bib penalties. Its sounds nicer than genocide.
3) Creating something similar to a Missionary but through time and money allows you to forceably convert a populace to your culture as is done with a religon. This may be too complex.