Here's the thing, I have something very specific in mind for this: I want to make Serbs and Croats around 2010's to think of themselves as one nation, the "Yugoslavs".
The POD can be immediately post 1918, or in the 1848-1918 period. If you really have to, you can go as far back as the Congress of Vienna. The main issue here is ethnic and cultural identity, how can it be made work.
1. I am only speaking here of Serbs and Croats. Albanians would be seen as "the enemy" as would Hungarians. Bosniaks/Muslims as "traitors".
2. Slovenia would for a long time be part of the country, but Slovenians would slowly begin to be seen as an "other" the more the common Yugoslav identity catches on among Croats and Serbs.
3. In my TL most of Macedonia never becomes part of Serbia, rather it goes to Bulgaria early on, so you don't have to worry about Macedonia. The Banat proper might go entirely to Romania (or not), but most of Vojvodina goes to Yugoslavia.
4. Some ideas floated aroun here which had to do with state evolution rather than culture would be to move the capital to Sarajevo in 1918 or have an early "Sporazum" in 1918 giving large autonomy the a united Croatian banovina and essentially to split Bosnia between Serbs and Croats. In time this will no longer matter so much if the 2 peoples truly come to see themselves as one.
Please remember that this will be before OTL's Ustase and Chetniks and both Croats and Serbs, so far have their identity in being Slavic peoples who fought the Ottomans. Also they both fought against the Hungarians and pro Habsburg monarchy in 1848. I think a large part of their becoming one nation would have to be in seeing Muslims (both Albanian and Bosniak) as well as Hungarians (perhaps also Italians) as the "enemy".
Also, Serbian and Croatian (and Bosniak) are essentially the same language, as they are based on the same dialect of South-Slavic. Coming up with a common "high language" might not be that difficult. (Literary Croatian is also based on Shtokavian; it's not based on Chakavian or Kajkavian.)