Hm well i don't know the specifics but have the soviets do a bit less well in rl. The wallies do a diversion landing in the Balkans.
Then have the partisans go the way the Greeks did and turn on each other (might have happened in real life for all i know), and the partisans are a lot more ethnicity motivated (you cant trust a Croat, the Serbs will oppress us etc things like that). And the big three might think that a new Yugoslavia may not be a good idea. They make a public announcement and are attacked by every group wanting there own country. And with some odd diplomacy Bulgarian style, alot of those states are approved.
The result, its just like a new Germany, filled with a bunch of states that cant stand each other.
I don't really know the details about the different partisan and independence and interest groups or if the soviets made it to Yugo in rl, and how successful a landing would be but i think this is a rough outline.