I reckon a Yugoslavia with Bulgaria included would be great, but calling it Serbo-Bulgaria or Bulgaro-Serbia might cause it to fall apart, seeing as the Croats, Bosnian Muslims (Bosniaks), Slovenes and Macedonians would feel like they're being Serbianised or Bulgarianised.

I think it's obvious that Serbo-Bulgaria would be name just for united principalities of Serbia and Bulgaria.

There was also project of Balkan Federation, creation of a Balkan federation or confederation, based mainly on leftist political ideas.