If Gavrilo Princip did not assassinate Archduke Ferdinand, what else, if anything, would likely have sparked WWI instead?
Nothing in history is inevitable, so if Franz Ferdinand had not been killed, it is atleast possible that World War I would never have broken out and we would be living in a much more peaceful and stable world today (with, incidentally, much better art). Possible, but not likely.
In the twenty years before 1914, war had nearly broken out between France and Britain over a tiny hamlet in Sudan, between Russia and Britain over a few mountain passes in Central Asia, and between Germany and France over influence in Morocco.
As the events of 1905 show, Russia was already on the brink of revolution a decade before the war broke out. Suppose it had happened, and Germany sought to take advantage by moving in to "protect" Poland and the Baltic states. Russia wouldn't take kindly to that, and already had a defensive alliance with France by that point.
Oil was becoming increasingly important, and both British and German companies were competing for access to Persian oil fields. I can see potential trouble there.
On a separate tangent from OTL, if Hitler were killed in WWI, what else, if anything, would likely have led to WWII instead?
My view- no Hitler, no WWII. Simple as that. I could see a communist Germany aligned with the USSR against Britain and France, but that's a rather different proposition.