Archduke Franz Ferdinand was the least warlike in the Austrian Kaiser's inner circle. He had been holding back the warmongers for years. Ironically and of course unknown to Prinip, Franz Ferdinand was also very much in favor of more rights for the Slavs in the empire. Ferdinand was also pushing for reforms of the monarchy and the military — modernizing with a view towards more democracy. His uncle the Kaiser was very much at odds with his heir’s ideas.

If it had been the much older Franz Josef who had been killed and not his young heir, it is very unlikely the ultimatum that lead to the start of the war would not have been made, and war - even if inevitable - would have been started years later and under very different circumstances. Meaning it would have been pursued very differently and had a different ending.