SO I actually made a thread on adding Slovenia about a month before this was made (so gotta disagree with OP about no one talking about it)
https://forum.paradoxplaza.com/forum/index.php?threads/inner-austria-adding-slovenia.1045991/
there really is room for adding a slovenian culture, but there are some debattable issues around it.
Adding Cilli/Celje province would be historically correct, having that part of untersteiermark (Lower Styria) as part of Krain is just wrong (even if they don't add slovenian culture we still need more accurate border in inner austria). Adding Cilli as a state would also be a nice touch, but having it use slovenian as primary culture would be some poetic licensing alright because even if the ruling dynasty might have had some slovenian origin (which I think is disputed) the county was a german county in the same sense that all those crusader states in greece has various italian or french primary cultures. I.e. Cilli would use Austrian as primary and Slovenian as an accepted.
I guess one of the issues of adding Cilli as it's own country is that it didn't rule a contigous land in modern Slovenia area but spread out castles and feudal holdings (look at the maps at wiki). Considering PDX treatment of the HRE it wouldn't be unprecedented though to make Cilli be an OPM controling all of Untersteierkmark (They did not hold the city of Marburg/Maribor in Untersteiermark though which I believe would be the largest city in the area)
So giving a tag for Slovenian culture, I don't think Kärnten/Carinthia would work, while it is the only Slovenian duchy in history the area of Carinthia was pretty germanized and the province of Carinthia should (as it has in game) have Austrian culture. So making a country for Slovenia when the province the country is based on isn't using slovenian culture, would be a bit strange so I propose using Carniola (Krain) as the tag for Slovenians. Kärnten could have a Carniolan core though as it was still significant slovenian minorities in the region.
Dealing with Austria littoral is, I guess one of the more delicate issues.
So this is a 18th or early 19th century map of Inner Austria. The interesting thing is while I can't speak about the internal borders within the historic lands, as shown here, the external borders of the various lands were almost unchanged for the entire EUIV timeline. (Lands meaning Küstenland/Littoral Carinthia/Kärnten, Krain/Carniola and Steiermark/Styria)
Some noticable things is how Istria is split between Austria and Venice. Also in game it is noticable that the Istria province (while being owned by Venice) has Trieste as capitol (which was a free city within Austria). Also Friuli is owned by Austria (in 1444 it was owned by a german noble dynasty as vassals of Austria), meaning for historical accuracy the city should become it's own province and not be part of Hungary.
The nature of Ethnicity in the whole littoral is highely debattable, and no matter what some people insist the area had italian (friulian or venetian) or Slovenian majority, I haven't seen any convincing sourcing proving either side right. Personally I'm leaning against having Görz use Slovenian based on reading that the town had a significant italian imigration after 1500. I would appreciate better sources though (and don't use 18th century Austrian census because that is proves nothinig for the 1444 situation)
The exact ethnic composition of Trieste is also vague but I believe the city itself should have italian as it was the elite culture. However as the province could be made to have more of Istria as shown on above map (the Austrian part of the March of Istria) it could very well be made a slavic province given the population would then likely be majority slavic (slovenian croat) though march of Istria could also be given to Carniola province but I believe that would make the provinces in the area too small.
The venetian part of Istria could then get venetian culture as romance speakers dominated the coast.
Fiume/Rijeka should also have venetian culture.
but I'm not aware of any tries to create separate states split on dialectal basis
Some time ago someone on this forum found an old british education book (19th century) online. It was generally really bad in terms of accuracy but gave an interesting perspective on how the brits viewed the world in the 19th century.
https://books.google.se/books?id=kb...6AEIKjAA#v=onepage&q=wendo-lithuanian&f=false
The book calls SLovenian
Windic and says it is spoken by several nations named after the countries they inhabit i.e. Carinthia, Carniola and Styria.
Nation I believe shouldn't be read as independent country but in the ethnical sense and country either denotes a
State but rather the notion of a
Land in the sense of a cultural and/or political body. This I believe (while not being entirely reliable) indicates a low level of common national identity among Slovenes. I see this could be one argument against slovenian culture in the game, ut really this is not strange circumstances durin the timeline of EUIV as cultures/ethnicity wasn't really that relevant back thn as it is today and slavic speaking people did inhabit the territories in question.