well, multi-culturalism worked prior to the 1800s in the Polish-Lithuanian commonwealth and in Austria-hungary. In Austria-Hungary in fact, the ArchDuke Ferdinand (whose death was the excuse that caused the local war between A-H and Serbia and was the excuse for the world war between Germany and Russia) was a proponent of a “United States of Central Europe” which would have Germans, Hungarians, Poles, Serbs etc. having equal powers. The Austrians had done with with the Hungarians in the Dual monarchy and could have extended it further.
I think as long as the Old Kingdoms stayed so stubbon and refused to change, it was going to be blown up one way or the other. There was so much tinder that any one of a thousand things would have provided the spark that caused the war.
Which was exactly why the Serbs wanted him dead. A US of CE with not overly dissatisfied minorities would have brought a screeching halt to the Serbian ambition of uniting all southern Slavs under their leadership.
Yugoslavia, of course, and we all know how well that turned out.