1) tame the avarice of the world's top 1-5% and replace it with altruism. Get those that have much to share with those beneath them, and set up infrastructure so more people can have a stable, reasonably comfortable life.
2) change the inner-most hearts of criminals everywhere so they could be released from jail, prison, half-way houses, parole, probation, etc. and not re-offend.
3) get drug developers to focus on cures for diseases, not long term management, so more people could be healed thru pharmaceuticals and other treatments and go on with their lives, without the need for ongoing treatment and staying in the patient/sick role.
4) get people all over the world to see their need for Christ. I think...I think a lot of times, the world blinds us to how much we need Jesus. I mean...sin, satan, self, death and the world...these are things we all deal with, and these are what Jesus came to set us free from...but I think the world is a major player in dividing us and keeping us from Christ. If I could, I'd miraculously give people all over eyes to see and ears to hear The Good News and get saved...de-program them, basically.
That's about it. I think my changes would probably end up costing more affluent people a lot of their $$$, but it'd pay off for everybody in the end. I think giving people all over eyes to see and ears to hear The Good News would be good for the church, good for people, but could potentially have a de-stabilizing effect on some societies, because Christians are different from the rest of the world, and Christian-izing societies doesn't sit well with many in and of the world.
Interesting question, btw.