Would an inclusion of Bulgaria have helped?
Yugoslavia always seemed to me to be a contest between Croats and Serbs on who ran it
My understanding is that most of the time, many Croats were more interested in preventing Serbs from running the show alone than being themselves in charge.
In a sense, Yugoslavia was bound to have some degree of Serbian dominance simply because of numbers and geography, as Serbs were the most numerous single nationality AFAIK (esp. considering that for a long time Montenegrines largely indentified themselves as Serbs ).
Bulgaria was always unlikely to ever join.
You could possibly have Albania joining in the late forties (Hoxha and Tito were on very friendly terms and a federation was seriously discussed before Tito split from Moscow). That would reduce Serbian numerical dominance considerably overall, although they'd remain a plurality I think (did not check).
A possible consequence could be a larger Albania including some Albanian majority parts of Macedonia, Kosovo and perhaps Montenegro (all of Kosovo is almost out of question) as a seventh republic in a somewhat looser arrangement from the start. Or maybe Albania gets split into a Gheg and Tosk republics, but I see such an outcome unlikely.