Slavic cultures are now in the Slavic culture group, effectively giving no penalties for having Slavic provinces as a Slavic nation.
Ever wanted to expand with Russia westwards instead of eastwards and have your conquests led by the Pan-Slavic ideology that drove the Russian Empire into kicking out the Ottoman Empire out of Southeastern Europe?
Now you can even annex these territories, with no penalty except for the religious one!
Gives a unique chance to form an all-Slavic nation and a unique playability to Slavic nations that can grow stronger much easier by unifying Slavic nations.
V jednote je sila!
Съединението прави силата!
Единство это сила!
W jedności siła!