Is it at all possible for a Bulgarian formed Yugoslavia to form? Perhaps after a Central Powers victory in ww1?
Not really, with the pre-existing Serb-Bulgarian enmity and the already entrenched ethnic identities of the various Slavic populations. I really don’t see Serbia willingly subjugating itself to Bulgaria and such I can’t see it forming.
OTL, Yugoslavia formed as a Serb-Croatian-Slovenian Union because each of those nations were too weak to go it on their own. Further down the line it became apparent that it was Serbian dominated, but by then it was “too late”.
A Bulgarian formed Yugoslavia to me could only come about by conquest and as such would be resisted by the constituent parts.