Budapest was at this point of time the fastest-growing metropolis in Europe. Like Vienna, it had more inhabitants in 1914 than it has now.
###
One often tends to forget that not only the Hungarians were included in "divide-et-impera" power-sharing agreements which placated most minorities. Poles were allowed to dominate over Ruthenes (and to enjoy a lot more autonomy than the Poles in Russia/Prussia), Italians over Slovenes and Croats in Dalmatia and Küstenland, Croatia had its own semi-autonomy.
The only really sore spots were
-the Czechs, but due to their impossible strategic and favourable economic position, they were improbable to rebel beyond nationalistic riots now and then (a game the Germans in Bohemia played just as well) and Parliamentary obstruction
-the Serbs with their irredentist agenda unreconciled by alliances with Austria-Hungary any more (unlike Romania & Italy)
I agree that a reasonably and ruthless constitutional reform at the right point of time could have dramatically increased Austria-Hungary's chances. But: the biggest obstacle stands in Budapest, a Gordian knot to be cut. I cannot imagine a way to successfully reform the whole monarchy without cancelling the "Ausgleich" and having a royal putch against the Transleithanian government and constitution.
The first sore spot could be placated with Czech autonomy under a Wenceslas-Crown. This would either include the Germans of Bohemia/Moravia-fully (a very strong minority), or these old lands would be separated à la 1938 (untypical for the Habsburgs); also dubious if "Upper Hungary = Slovakia" was was to be added to this crown. IMHO a combination of both ideas would have been best.
The second sore spot is harder. FF was dead set against annexing the Serbs, despite his sympathy for a South Slav crown. So I assume that by granting the Croats their own state, expanded by Bosnia-Hercegovina, that implied that they get the responsibility to take care of the Serbs. Not Vienna's business any more. Could have become nasty over time, but not as nasty as OTL!
As you can see, this is all very complex already. And now you want to expand? IMHO, that's hard, and nobody reasonable within the Hofburg was interested in that idea in the 1910s. Yes, Serbia and Montenegro would look neat annexed on a map - but they are the Balkan's Vietcong.