In their agreement about Serbo-Bulgaria it is not specified, but Serbia led policy of liberation of Serbian and South Slavic people, so I supose it would be eventualy.

Austrian map of "Jugoslawisches Reich" from 1915.

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.