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There is simply no idea or concept that can unite a Bulgarian and a Slovene to live in a same state before 19th century and panslavism. Something like Podunavya would make some sense, as it would be South Slavic name for "land below Danube" which is essentially former Yugoslavia + Bulgaria. Some of the most defining characteristic of the region are Balkan mountains (to a lesser degree) and river Danube which really defines the entire region. If you absolutely must have a unification, go with geography. Literally anything else, such as "Carpathia" for the region just above it (in terms of coming up with a geographic name for a unification, which is nonsensical otherwise), is better. No unification is better than made up fantasy unification. It never became a thing in the rest of the Balkan because it wasnt part of Austria/Austro Hungary OR it already had their own nationalist/linguistic movement. In history this was a group of linguists who tried to unify as a linguist movement in order to standardize and preserve Slavic languages of people that were in Slovenia and Croatia, and prevent Austrian aggressive policies of assimilation that aimed to destroy these nations and make everyone German. We (since i am from the region) dont see ourselves as Illyrian, this literally doesnt mean anything. This is a nonsense idea promoted by Death & Taxes and then M&T which makes no sense whatsoever.
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