PoD: Entire Yugoslav Partisan command is obliterated in an attack (basically Tito dies) - Butterflies ensure that the war lasts to early 1946.
The war in Yugoslavia becomes much more messy, the Red Army props up a now rump communist party, but with a twist - unlike OTL, most of the leadership now consists of Serbs. Instead of the 6-republic division, the new state is divided into 18 federal states with little regard for ethnics, similar to the previous division.
After the end of the war, which was even more terrific, the relocations start. This has Stalin's approval and is modeled after his deportations of the Volga Germans, but weirdly enough, always into Serbian majority areas. With never more than 1 family resettled into the same town. The western allies start to question these methods, suspecting nefarious intentions. Just like the expulsion of the Germans from Eastern Europe, they found themselves unable to justify the criticism of the mistreatment of the peoples they were demonizing just prior.
Serbs aren't spared the relocations either - deliberately being sent into distant lands, but always at least 5 families into one settlement. Few noticed this. Then Cyrillic became the sole official script, which was presented as a move to show closeness to "our Bulgarian and Soviet friends". The Soviets instituted Cyrillic in Romania almost immediately after the war.
The line of thinking of the communist party was as follows - there are too many nations in a small area. If the state is to survive, they must be coagulated. Knowing that creating a Yugoslav identity would not manage to do the job in time, they elected instead to just pressure everyone into assimilating into the plurality of the country - Serbians. The new states are drawn up to always include at least one Serbian minority area. The entire state then would with no exception be presented never as a Serbian one, to lull the populace to feel like they're in charge. In reality, the top echelons of the local government were with no exception almost completely Serbian. All the state capitals always are in nonserb cities if they can be.
The communists found themselves pretty unpopular - solution? Decision to stop the oppression of religion, which affected the Serbian Orthodox Church disproportionately. Vatican was seething even before this, but now is basically having an aneurysm, USSR screamed revisionism. Yugoslav-Soviet split. So the Yugos invite western troops. USSR stands down. Chetniks, which were never undefeated in full and were becoming increasingly popular (and the central authority turned a blind eye to them) are placated and satisfied. The innumerable settlers are pretty much forced to convert at this point - internal movement was restricted to everyone, and now nobody wanted to be in a mixed-religion marriage.
Propaganda machine starts working, working on making Catholic Herzegovinians and Dalmatians feel guilty and judged by their ancestors for not undoing the mistake of their grandfathers of conversion (these recent conversions are a eternal meme here, Serbs make fun of Croats in Dalmatia and Herzegovina that they don't dare go into their attics because grandpa's icon is there which makes them feel bad). Similar things are undertaken in Bosnia. It has a moderate degree of success.
Enter 1959. Angered by the now blatant discrimination, Croats, Bosniaks and Slovenes start an uprising. Big mistake - state security puts neonazi agitators in the crowds, entire movement branded as "new ustaše", which makes everyone extremely angry. The rebellion is subdued with extreme violence. Yugoslavia placed under sanctions. In 1968 after the crushing of the rebellion and starting an even bigger wave of population movements, the government officially declares Yugoslavia defunct, and changes the name to the Socialist Republic of Serbia. Western powers threaten invasion - so the government allows Soviets to place their nuclear missiles in Yugoslavia. With the Caribbean nuclear crisis (happens as OTL) still fresh in memory, the west backs down. Serbia becomes further isolated, not very willing to cooperate with the USSR (which is starting to undergo similar changes to Serbia - a lot of Russification is going on even more than OTL). Government shifts economy to an autarkic system. It goes with a few hiccups.
Nothing of relevance happens all the way to 1978. In 1978 the plan is declared a success, and the federal units abolished completely.
In 1984. USSR starts seeing upheaval over the bad state of the economy. Reforms are being prepared. Similar thing starts happening in China.
In 1985. the economies of all communist countries start going into severe recession due to inability to compete with the Western one. Both the USSR (changing the name to Russia, much to Baltic chagrin), and China turn more liberal, almost identically to China OTL.
In 1986. tired of the decades of isolation, Serbian citizens launch protests to do something about the neverending sanctions. Referendum on reinstating the monarchy is held informally by the Chetniks. The proposal wins. Despite being very informal and not up to standard, the government declares that the will of the people has spoken - King Petar II returns to Serbia. Sanctions are lifted, as the west can't do anything anymore, and their demands from 1967, which called for the immediate return of all families to their homelands carries no more weight - the assimilation is almost total.
States, with census data from 1977:
1. Croatia, capital Zagreb. Croat plurality in 1977. More and more call themselves Croat as mere regional variant of Serb., Called "New Croats" by the actual Croats.
2. Istria, capital Reka. Croat plurality in 1977. Slovenes assimilated into Croats at this point.
3. Pannonia, capital Osek. Serb majority 1977.
4. North Serbia, capital Belgrade, which is the national capital. Serbian majority in 1977.
5. Krajina, capital Gradiška. Serb majority 1977
6. Lika, capital Cazin. Serb plurality 1977.
7. Srem, capital Vukovar. Serb majority 1977.
8. Bosnia, capital Vrhbosna (Sarajevo). Serb plurality 1977.
9. Braničevo, capital Petrovac
10. Podrinje, capital Srebrenica. Serb majority 1977.
11. North Dalmatia, capital Split. Serb plurality 1977.
13. Raška, capital Ras (Novi Pazar). Serb majority 1977.
12. Golija, capital Hoča (Foča). Serb majority 1977.
14. South Dalmatia, capital Dubrovnik. Serb plurality 1977.
13. Raška, capital Ras (Novi Pazar). Serb majority 1977.
15. East Serbia, capital Niš. Serb majority 1977.
16. Metohija, capital Peć. Albanian plurality 1977.
17. Old Serbia, capital Skoplje. Serbian majority 1977.
18. Pelagonia, capital Bitolj. Serbian majority 1977.
However, by 2000, if they still existed, every state would have a Serbian majority. Serbia, along with Poland has one of the quickest growing economies in 2016, and is a part of the Mediterranean Union. Militarily neutral, Austria and Hungary are NATO members, neutral meeting ground between the European Union and the now increasingly powerful and maybe even more prosperous Eurasian Union, nicknamed "2nd Switzerland".