There won't be anything directly analogous to Franz Ferdinand that starts WWIII. The extremity of total war post-WWI makes it unlikely that one politician's assassination would drive any country to start nakedly posturing for war the way Austria-Hungary was towards Serbia. And, of course, the current geopolitical environment is divvied up between the alliance networks of a declining hegemon and the resident upstart, instead of a group of roughly equal Great Powers.
WWIII is more likely to be set up WWII-style, where a rising power keeps flexing their diplomatic might to demand concessions from their neighbors, until they take a step too far and the rest of the world intervenes. Off the top of my head, a dispute between India and China over water rights in the Himalayas could trigger war. I'd list the annexation of Taiwan as a potential starter, but looking at how we treat Eastern Europe and the slow annexation of the former Soviet states by Russia, I give it even odds the western world just rolls over should the ROC ever be invaded (although, unlike Eastern European shitholes, Taiwan does produce some pretty critical goods, what with all the chip manufactories, so maybe there'd be more concern).
