If you could choose just one thing to change about the world, what would it be?

The world would be governed by a single just leader, because dictatorship is the most effective and fair form of government if there is a just leader making the calls. No media coverage (press conferences) would be necessary, so the leader would live out their life in privacy and thus have the ability seek out a suitable successor with ease. This may seem implausible since different people have different views of morality and justice, so this person would have to be extremely knowledgeable and capable in all areas—genius enough to make decisions and legislation that would improve the quality of life for the largest number of people possible, if not everyone. So yes, somewhat implausible.