Personally, I'm going to opt for a revamping of the world's educational system. The system is set up to keep young people out of the work force as long as possible to concentrate wealth in the hands of older workers. Notice how a college education is practically mandatory now and the sheer number of students seeking higher degrees has increased dramatically; work shadowing and apprenticeships are a better solution and wouldn't require a student to mortgage their future to put even more money in the hands of the ruling generation. It would also solve the problem of who gets an education - females, the poor, etc - since everyone would have the same opportunities to learn a chosen profession.
What do you choose? World hunger? World peace? Global warming? Bring back the Do Do birds?