I’ll be quite my previous post on the topic, in which OP specified a PoD after VE-Day.
Tito decides to never set up the Territorial Defense Forces. The only military force in Yugoslavia is the JNA, and the general staff ensures that no Croat-majority units are stationed in Croatia, no Slovene-majority units are stationed in Slovenia, and no Bosniak-majority units are in Bosnia. The constitution also doesn't include a right to secede. Bosnia and Herzegovina has the Serb areas and the Croat areas each become autonomous regions like Vovjodina, and Krajina is also an autonomous region. This should be enough to ensure that any Croat or Bosnian independence attempt will become a far bigger clusterf*ck than OTL, and that no secessionist will have a proper military. There also needs to be some way to obliterate Kosovar separatism. Enough brute force should do the trick as long as both the West and the Soviets look the other way, or one of them actively supports Yugoslavia.
Other posters in that thread pointed out the republics had well-armed paramilitary police forces. Those should be replaced with a federal police force along the lines of the FBI, with local police armed only with pistols and shotguns. After 1945 though, I don’t think there is any good way to bring Bulgaria into Yugoslavia and have it remain stable.
I reject the idea that decentralization would save Yugoslavia. Czechoslovakia was more decentralized and still split, but peacefully. Serbia and Montenegro was also more decentralized than Yugoslavia ever was and still split. Over-centralization is obviously a problem, but so is a lack of centralization.
The Kingdom could have possibly survived in its OTL borders without decentralization, but it would need to develop a healthy parliamentary democracy. Banning ethnic parties would backfire, so they should be tolerated. The state should be secular and not favor the Serbs, but it shouldn’t favor the minorities either.
If decentralization with a “Banovina of Croatia” and a “Banovina of Slovenia” is necessary though, the borders should be drawn to favor Slovenia at the expense of Croatia. This would weaken the Croats, the greatest threat, and make Slovenia dependent on Belgrade for protection from the Croats.
For Royal Yugoslavia, finding some accommodation with the Montenegrin royal family would also help, but isn’t strictly necessary. Maybe they could be demoted to Grand Duke and left as a figurehead, or left as a king and King Alexander can promote himself to Emperor.
Bringing Bulgaria into Yugoslavia is rather difficult, and would probably require a PoD before the First World War. Maybe Austria-Hungary attacks the Balkan league during the First Balkan War, starting the Great War early with the entire Balkan League in the Entente. The experience of fighting alongside each other brings the members closer together. Maybe they recognize the value of linking their economies and form an economic union. Eventually this union becomes a full federation. If the union includes Greece though, it would be a Balkan Federation, not Yugoslavia.
Technically done in the 1. Yugoslavia. But it was mostly done to weaken Croatia, and this Yugoslavia didn't survive.
There was this event called the “Second World War” which was the main reason it didn’t survive. We can only speculate how the Kingdom would have ended up without the Second World War or any other major conflicts.