I don't think a restoration of the monarchy would be a solution. Even when it existed, the monarchy always had problems with non-Serbs.
The real issue was the regional economic disparity between regions in Yugoslavia, and the wrenching economic pain in 1989-1991 which overcame residual pan-Yugoslav loyalties. You need to address the economy. If it did not get as bad as it did in 1990, then it is possible for Yugoslavia to get past this critical period. If it holds together to 1994 or so, then the pressures leading to ethnic separatism would decline and the country could stay together in some format. I think the premiership of Yugoslav Prime Minister Ante Markovic is important. He was the one who had to navigate Yugoslav from a socialist to market economy.
In retrospect, the West probably insisted on too severe austerity measures in exchange for cooperation. Yugoslavia just could not handle the stress of that with their internal divisions. Another, probably more serious, problem was that somehow Serbian President Slobodon Milosevic somehow forced Serbia's main bank to distribute $1.8 billion to increase Serbian government wages and pensions in order to win re-election to the Presidency of the Serbian Socialist Republic. By doing so, he revived the high inflation that Markovic's policies had successfully defeated with his austerity measures. With the return of inflation, Markovic now had nothing to show for it. In fact, Milosevic had pretty much did everything he could to undermine Markovic's reform policies.
This is not to say that if you got rid of Milosevic, things would have gone smoothly. His opponent in the December 1990 elections was even more nationalist, and the Slovenians and Croatians (Markovic was a Croatian too) were moving towards declaring independence. So another critical piece would mean some kind of new constitutional arrangement that could be agreeable (albeit barely) between the Slovenians and Croatians and the Serbs. This would have to be done in 1989 or early 1990. If this could be done, then the steps taken in spring 1991 to split from Yugoslavia might not happen. And as the economy improved, gradually the tensions would be lessened.
These are not insurmountable problems, but they do require a superb caliber leadership and fantastic timing for it to happen at the right times. It's far easier to say "Stop Milosevic from raiding the national Serbian bank" and "achieve constitutional reform in early 1990" than it is to actually do those things.
