The challenge: with a POD after 1974 (the year the last Tito-era consitution was enacted), have Yugoslavia transition away from communism without breaking up and without any major ethnic violence.
Stopping the Reagan-enforced, sorry, encouraged economic reforms that absolutely ruined the economy would do a lot to help. Also, make sure that remarkable rise from mid-level bureaucrat to president of Yugoslavia of a certain S.M. doesn't happen.
There have been a few threads about the break-up of Yugoslavia just a few weeks ago. If someone has the link...
I agree that someone like Ante Markovic should have been able to politically rally those elements of Yugoslav society which didn't wish for a break-up. Maybe a slightly earlier liberalization would help. In an ASB-scenario, I proposed a Spain-like transition via the way of monarchy.
It would also help immensely, if the Yugoslav constitution didn't create such large armed forces under command of the single Republics. I am not promoting a better outcome for the Serbs, but maybe a deterrent for the separatists.