As an unified large state it would run into several problems.
First of all this nation is way too big to defend itself. Russia alone has problems defending their far east territorities. There's also problem with Orthodox East using Cyrilic writing system while West uses Latin, however it can be simply solved by just teaching 2 alphabets at school. The language isn't big problem either, slavic people can widely understand each other. There is also Interslavic language existing, that is mix of all slavic languages, it has been created to make it easy for any slavic person to be able to learn it without any problems. However the second most widely spoken language would be Russian and third Polish.
https://en.wikipedia.org/wiki/Interslavic_language
Can you tell what does that mean ?
"Otče naš, ktory jesi v nebesah,
nehaj svęti sę imę Tvoje.
Nehaj prijde krålevstvo Tvoje,
nehaj bųde volja Tvoja, kako v nebě tako i na zemji.
Hlěb naš vsjakodenny daj nam dneś,
i odpusti nam naše grěhi,
kako my odpušćajemo našim grěšnikam.
I ne vvedi nas v pokušenje,
ale izbavi nas od zlogo."
It's Christian prayer "Our father" in Interslavic.
Other problems that nation would run in, would be most likely West EU and United States being hostile to such state, and people would probably compare it to Soviet Union reborn anyway. This new country would also have to create new "Slavic" identity, familliar to what they have done in Yugoslavia. (Around 50% Bosniaks have identified as Yugoslavs, I lost the source, but I have read about it somewhere on wikipedia).
In my personal opinion, new Slavic identity could be encouraged. But, not forced. Original respective nation culture should be conserved, and not replaced with Slavic.
Here's map of all Slavic nations, however Sorb minority in Germany and Russian majority in north kazachstan wasn't included in this map. To connect South Slavs with North Slavs the Romanian coast would have to be annexed.
Anyway and lastly, such union would be hard to form since hostilities between Slavic nations.
Croatians hate Serbians. (Yugoslav wars etc)
Serbians hate Bulgarians (Second Balkan war etc)
Poles hate Russians (Occupation, several wars, massive murders of civillians)
and so on and so on.
But, if any hostilities would cease and nations would try to apologise each other and start to become friendly, and maybe try to refund any damage, such alliance would be possible. But, I don't expect the union to create any time in future. Russia tends to be nationalist, and anti-west. Serbs want to restore Greater Serbia. And whole Pan-Slavic movement isn't that popular of course due to these historical disputes among nations.
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