How can WWI happen with a neutral Austria-Hungary? Where would it most likely have started?
hardly possible, 1905 Russia was knocked out by Japan, but Germany nevertheless didnt start war against France over Marocco - so it is very difficult to find casus belli for an european war without the Balkans (and than A-H. cant remain neutral, thats pretty sure).
Okay best bet: Balkans dont blow up in 1910s, Russia is getting stronger and stronger - it starts the Great Game 2.0. now with a growing industrial base and RR to India/Persia. France decides to back the Russian plans and lends lots of money as it still wants revenge for Alsace-Lorraine.
Franz Ferdinand gives the Slaves more autonomy and manages somehow to contain Serbia. He wants to establish strict neutrality (similar to Switzerland) because of the many ethnic groups ans potential internal conflicts.
British Empire starts sending troops to India/Persia but cant compete with Russia, so negotiates alliance with Germany. After several Russian blackmails, the British Empire is forced to allow the Russians grabbing large parts of Afghanistan and Persia. So Britain starts supporting Polish "terrorists" who fight agains russification policies in order to weaken the "Big Bad Russian Bear". The assassination of the Russian prime minister with weapons from England is casus belli, Russia immediatly starts invading India + Persia.
The German Empire is afraid of the growing Russian giant and decides to declare war as long as there is a chance to win the war in accordance with the british-german mutual defence agreement. Japan being a British ally, joins the war in order to prevent Russia taking revenge for the 1904/1905 war.
France on the other side wants to grab Alsace-Lorraine and attacks Germany, hoping for an easy win as Germany has to deploy most of the forces to the east. Have fun continuing