There are four powers that can most obviously and directly take advantage of this: Russia, Italy Serbia and Romania. For Russia, the commitment to help the south slavs break lose from the Hapsburgs would be put to the test. Russia and Serbia are probably the most dangerous should they attack. A big factor here is whether Croatian territories go with Hungary, secede from it to stick with Vienna or simply go for their own full independence right out of the gate. And who knows what would happen in Bosnia.According to the Austrians it might be. But Hungary was legally its own state, its inhabitants had their own citizenship separate from the Cisleithanian citizenship. And since the Ausgleich had to be periodically renewed by both halves of the Dual Monarchy, that implied that Hungary had the right to refuse to renew it or accept a revision to it and take back all of its sovereignty. Regardless of the legal situation though, it would be extremely difficult to prevent Austria’s enemies from taking advantage of the situation.
Italy would be tempted to attack but I personally think that it would not do so unless Austria looked like it was definitely going to fall apart and they could be pretty sure they would get the Adriatic territories. If they did attack early though, they could do enormous damage and the empire might need substantial help from Germany to withstand it.
Of the four main external threats, I think that Romania would be the perhaps least likely to cause trouble. The Romanians had more of a problem with Hungary, of which Transylvania was a part, than they did with Austria. Since Hungarian independence very likely means Romania still doesn't get this land, I don't see it coming to Budapest's aid too readily. Maybe the Hungarians would eventually strike a deal out of desperation but by then, it might be too late.