"Culture" is far too vague, and that vagueness makes the "change culture" button sound like genocide, I think splitting culture up into two separate entities will make things more realistic and historical as the prevalence of many languages changed greatly during this time period, while the people and cultures in most areas typically stayed the same or similar.
1. Language - Self explanatory, in 1444 there were many many dialects and languages, particularly in places like Italy, France, Germany, etc, changing a province's language would work similarly to how culture conversion works now, except much less expensive, less time consuming, with a revolt risk during conversion, and with a thirty year cool-down, similar to the religious conversion cool-down introduced in 1.12. Languages can only become accepted if their provinces provide 20% (10% with Humanism) of your total development.
2. Ethnicity - The actual people, their culture, etc, this will remain static, only changed by rare events or decisions, such as Russian migration into former Horde territory, or the Ottoman conquest of Constantinople. An Ethnicity only become accepted if one of three conditions are met. The first requires them to provide 20% of your total development, Humanism will reduce this to 10%, they will become accepted regardless of language spoken if this condition is met. The second requires them to speak your language, even if they only provide 0.1% of your total development they will passively become accepted after twenty years if all your owned provinces of that ethnicity speak your national language. The third requires owning all provinces inhabited by a specific ethnicity and having them all speak your language, fulfilling this condition will have them become accepted instantly.
Revolt risk - A province with an unaccepted language & ethnicity will have a high revolt risk, enforcing your language will reduce the revolt risk by 2/3 and remove most penalties, however during the conversion process you will see a increased revolt risk, similar to when converting religion.
Example of province with unaccepted language & ethnicity:
+3 Revolt Risk
-10% Manpower
-20% Trade power
-20% Tax/Production/Trade Income
+10% Development cost
Example of Province with just an unaccepted ethnicity:
+1 Revolt Risk
-10% Manpower
But what about cultural unions? They will be split into minor and greater unions, for example France is a lesser union, all French ethnicites see France as their home land, automatically making them accepted, but the various French dialects and Occitan will only be partially accepted, giving you only half the regular penalties. An example of a greater union is Hindustan and Ming/Qing China, both of these will not only accept all associated ethnicites, but all associated languages as well.
Example scenario: France starts the game with only a small portion of it's nation speaking French, part of France's long term goals will be establishing French as the national language by replacing the various dialects in the north, and Occitan in the south, with standard French, as it did historically during the later parts of this period, they could potentially be aided in this by their second idea, French Language in all courts, which could potentially provide 20% cheaper language conversion in place of the current benefits.
Possible new feature: Diplomatic Language, unlocked at Diplomatic Tech 16, and based on the presence of a dominate diplomatic lingua franca in Europe during the later half of this period, it would provide a minor boost to relations and the speed of relation improvement between nations that share the same Diplomatic Language, part of expanding your influence in the mid-late game could be making your language a world language, similar to English today and French in very late parts of this period.
Please tell me what you think of this!
