Main difference there would have to be that the Germans had their Holy Roman Empire+East Francia, the Italians had the Romans, the Ostrogoths, and the Lombards. The Scandinavians had their Kalmar Union (Denmark-Norway and Sweden-Norway were also present). Those regions had been unified in some form prior to 1444. Geographically and logistically speaking, such nations were logical (all continuous, major population centers within reasonable traveling distance so communication would be feasible from one end of the nation to the other).
Contrast that with a unified Slavic polity. Neither has one of the scale proposed ever existed, it also is non-continuous (south slavs being cut off from the rest) and seems just too large geographically to be administered effectively (from Moscow to Prague would be the length of the whole German culture group alone (over 1900 km), which was a bit of a problem for pre-railroad administration historically). Moscow to Belgrade is even worse due to the Carpathian Mountains making travel even harder (the alternative is going around the Black Sea coast, which is quite an additional distance.
The PLC had no such dreams of uniting all Slavs (as far as I know) and Russia didn't start with that until the 1860s, which is later than any other pan-nationalist movements (which all began in the direct aftermath of the Napoleonic Wars, which are in the EU4 timeline).
On the other hand, Yugoslavia (or some equivalent) is not, in my mind, quite a ridiculous proposal. Yugoslavianism started up alongside pan-Germanism and pan-Scandinavianism (1830s) and the region had been united under various local polities (Serbian Empires, Bulgarian Empires).
TenshiN
1) Yugoslavia (united Southern Slavic country) would make sense. Small states such as Serbia or Bosnia alone would not resist the expansion of the Ottomans, or the Habsburg Empire. United give you advice. I know that Yugoslavia was founded in 1919, and the idea a bit earlier, but this state would be a useful and interesting solution. In the game, it is possible to create italy and germany that were created almost half a century after the end of the game.
2) Slavonic Empire, sounds a bit like Pan-Slavinist, but that would be an interesting option. If this does not fit you then I will add:
* Iberian Empire
* The Baltic Empire
*Roman Empire
* Northern Empire (Scandinavia, Britain and Ireland)
It will only be power, and playing them, or against them would be interesting.
3) Sejm - just as I described an interesting system of power.
4) New cultures - may seem small and insignificant, but will be mixed in countries where they will be persecuted
5) Transfer of Slovak and Pomeranian culture is necessary to make this game consistent with history. And on the map it will not look strange.
Roman Empire exists, Northern Empire is ridiculous (Cnut's Empire fell apart immediately after he died and the likelihood of reunifying that was null. England post-100 Years' War avoided continental obligations and the Scandinavians were in no position to invade England again). Iberian Empire would've worked had Miguel de Paz not died but, as the Habsburgs showed us, the Portuguese might not have accepted Castillian dominance so easily. Baltic Empire is...what? Unified Balts? That was never on the drawing board to begin with (Estonian isn't Indo-European while the other two are, Lithuania was tied up with Poland, the religions later got muddled with Lithuania staying Catholic while the other two went Protestant) and could hardly be called an empire either (too small a population to hold any much power, let alone avoid annexation by its numerous powerful neighbors).
Most formables are based on nations that had existed/would exist in the EU4 timeline (or would've been logical had some things gone another way). Hence why the Austro-Hungarian Dual Monarchy and Belgium don't exist in EU4.
Poland already has its unique government. If anything, we need fewer country specific government forms and more government forms to choose from for all nations (because why is the harem limited to the Ottomans? Why is militarism a Prussian exclusive when any highly militarized nation could've potentially done that? Why is England's constitutional monarchy unable to be replicated by any other nation?), at least in my view.
Slovak and Pomeranian? Eh, sure. They'll just get culture converted by 1500 is the issue (which is why Karelian was moved to Russian, Welsh to English, Breton to French, Basque to Iberian, Miao to Chinese, etc.). But I've no qualm with it.
