A crazy idea, but we might need something like this:
The Italian party system disintegrates earlier, a right-wing government including the neofascists comes to power. Yugoslavia fears that Italy might take too much advantage if it falls apart, so the Slovens and Croats stay nolens volens together with the Serbs.
It might also help if in Germany Oskar Lafontaine is elected in 1990, since the Kohl-Genscher government acknowledged Slovenia and Croatia. However, this will be difficult - Kohl was seen as the maker of German unity, and Lafontaine is too much on the left side. So unless there's a scandal right before election (Kohl talks with Lambsdorff about the question that taxes will have to be raised after election, which they promised they wouldn't, and forgetting that the microphone is on), no chance.
Yes, I see it like that - a threat from outside could help to keep the country together. Esp. in a time when everything changes.