Global Warning would be one, but really people all getting along no matter what race or background.
This is off-topic but Global warming is a LIE! Its called the sun, because very planet in our system is heating up at exact same rate as the Earth. So unless there are cars and factories and air planes on Mercury, Venus, Mars etc. its not us that's causing it.
Mine would be for some genius to create a machine power source that ran on CO2 and all that bad stuff and it's by-product was Oxygen and all that good stuff, and it was cheap enough to distribute around the world.
i would change the world to make it that you are born with all you can learn. you don't have to go to school you already know everything you will ever learn in your lifetime
If I could change the world I'd probably want to instill proper equality of opportunity. I have no problem with inequality, but it really bothers me that people who are born poor, are likely to die poor, regardless of their talents or abilities.
That's why I'd like to give everyone on the globe an equal chance to succeed, whatever that may entail education/welfare wise.