Balkan Federation was an interesting concept that had a lot of potential, and that had it's fair share of supporters.
Before the Serbo-Bulgarian war of 1885, there were talks of a union between the two principalities. I agree with Sturm that that specific war was one of the most foolish wars that was ever led in human history. Serbian government attacked its ally Bulgaria because of some promises made by its enemy Austria-Hungary. Foolishness. That war created a lot of resentment towards Serbia in Bulgaria. And justly so.
First Yugoslavia was some kind of Balkan Federation, but it was intentional made to leave Bulgaria out of it. And to keep it out. Adding another populous nation with strong army was not a welcome idea in Serbian court. There was also a lot of anger towards Bulgars because of their conduct during The Great War.
Socialist Yugoslavia was an updated version of the first, adding a little bit of territory to it, and leading (until 1948 at least) very successful union talks with Bulgaria and Albania. Of course, the union was put forward by USSR, but they were also the one to make an to it. One of the reasons they wanted Bulgaria in the union (even tho Bulgaria was an enemy of Yugoslavia in WWII) was to use them as some kind of Trojan Horse against very independent Tito. Bulgarians were much more submissive than Yugoslavs. You got to remember that Yugoslavs and Albanians were the only nations to liberate themselves without any major help from anyone.
Comintern's idea for a Balkan federation at it's biggest looked like this
Also, a few of you here made a mistake. Yugoslavia went further with union talks with Albania than ever with Bulgaria. There was a customs union with Albania, Serbo-Croatian was a mandatory language in schools, and Yugoslav national holiday (29.11) was proclaimed to be Albanian's too. And when Yugoslavs started building new government buildings, they made plans with one extra room in it - for Albanian delegation.
Of course Tito-Stalin split happened in 48, and communist around Yugoslavia (foolishly I might add) choose Stalin over Tito. That cemented the faith of communist party of Greece in Hellas, and third world status of Albania.
Romania latter on looked up to Tito's independent policies and started making moves without consulting too much with USSR.
1992. at the beginning of civil war in Yugoslavia, Slobodan Milošević, president of Serbia (then member of Federal Republic of Yugoslavia) went to Greece and proposed union with Macedonia and Greece. While all three sides agreed it's an interesting idea, it didn't gain much support because of the ongoing war in Yugoslav republics of Bosnia and Herzegovina, Slovenia (for brief ten days) and Croatia.
I think that nowadays talking about Balkan (con)federation in its traditional sense is a little bit outdated. With half of Balkans already in EU, and the rest to follow in the next decade there is no need for creating another political entity.
Also, I think that Croatia has in its constitution written that they can't join any other pan-slavic union.
But if it were to make another union, I think that it ought to be built upon Byzantine heritage instead of language. Because in "Byzantine" Balkan federation you wouldn't have a problem with nationalists that you had in SFRY where everyone claimed everyone and everyone was afraid of their identity. Bulgars, Hellens, Serbs and Romanians (why not) are all very different yet very similar culturally, and there wouldn't be fear among them about assimilation. And there wouldn't exist any sort of historical revanshism, and it would be a mono-confessional state (with Orthodox Christianity being the dominant religion). I think that mono-confessionalism would be a big thing. Yugoslavia's Achilles' heel was dominant influences of THREE major religions (Catholic, Balkan form of Islam and Orthodoxy) whose members often looked outside of Yugoslavia for everything, from financing to advice and justice, and barely recognized Yugoslavia as their homeland.