International federations are doomed to fail from the start because of high national pride. Basically, no country would feel satisfied to be under the foot of a bigger partner. You could say the Slovenes and Croats wanted to leave after Serbs got a little power hungry. Then again, the nationalist pride started growing in those countries long before the war and the Milosevic's power hungry politics was just the final drop that tipped the glass. The resentment of ex soviet states towards Russia is exactly because Moscow dictated the internal and external politics. If there is no order and balance in a federation then it won't last and will hurl the federation into a bloody conflict.

But how do you make countries with population of over 140 million like Russia be equal with small countries like Montenegro that doesn't even have one million altogether? It is an enigma because the strongest and most numerous will always tend to hold the power and are more likely to hold high positions in the government and military, whereas the smaller and weaker ones wish for equality. So either side has to sacrifice national pride in order for it to work and that would mean believing in something greater than your nationalist belonging. It would take great propaganda to convince people that we're all equal as Slavs and not make national divisions in a new country which is pretty much impossible in this day and age.

Also, I've read somewhere Bulgaria was considering joining Yugoslavia during the Cold War era, and there were actual meetings between officials, but because of the strong Soviet influence it failed and the idea was dropped. Perhaps with Bulgarians in the state, the Serbs and Bulgarians would've been able to keep each other in balance of power as these countries are similar population-wise.

One more good quote:

"Such a confederation must be one of equals. The weakest must come to the conference table with the same confidence as do we, protected as we are by our moral, economic, and military strength. "

Dwight D. Eisenhower