If I had one thing I could change in this world, I would remove ignorance.
It would cause many problems to be solved nearly immediately. Religious conflicts would have light shed on them. People would accept gay rights. Everyone’s issues/beliefs would be understood by everyone else. It would cause a massive progression towards a perfect civil rights system. The world could become one country due to understanding everyone’s thoughts and their differences and respect them.
The known truths would be known by everyone. Mathmatically, scientifically, socially etc.
Each person would be able to still act independently, but with the knowledge of everyone else in the world. This would enable the world to discover and create new things at an insanely fast pace, progressing the human race towards perfection.