Bulgaria was indeed completely obsessed with returning to the borders of St. Stefano and wanted Macedonia quite badly, so maybe there would be some sort of German dictate in that regard. However, Hitler was pretty content with keeping Yugoslavia as a source of resources and stability in the region and since, as you said, Bulgaria didn't even declare war on the SU, I don't really see why he would prioritize Sofia over Belgrade other than to quicken the defeat of Greece.
Tito's communists were bat-shit insane when it came to Istria, especially Trieste. They wanted the city very, very,
very much as it was another source of prestige for their new regime. They almost wanted to go to war with the Americans over the place and when Stalin pointed out the fact that the Yankees had the, you know,
atomic bomb they merely shrugged and said
so what ?
A non-communist Yugoslavia who manages to avoid directly participating in the Axis war effort (difficult), manages to switch sides at the exactly right time without facing some sort of Soviet presence within its borders (very difficult) could perhaps manouvre itself into a position where it might be able to successfully claim Trieste and some undetermined bits of Carinthia and Styria.