Economic Collapse
What do modern people wish for? What would make our day. How about never having to change batteries in your phone. How about never charging anything again? How about no more gas for your car? Well these things are inevitable. Think about it. If you go back 100 years and told people they would grow old in a world thick with air planes, orbital travel, robots building cars, cars in every garage. What do you think they would say? Well, some one who was alive in 1903 might have been old enough to read about the Right brothers flying in a powered kite. Then, they may well have lived to watch humans walk on the moon, only sixty some odd years later.
Can you imagine how much people would pay for the fountain of youth? It would stop all other trade instantly. Nothing else would be worth anything. The next big thing I believe will come out of stem-cell research. I think we will fall upon the fountain of youth and not know it for a long time. I believe it will happen in China because of their zeal at the moment.
Maybe the next thing will be a power source. Think about a commodity of that magnitude simply sputtering away into the either. That would make nearly one third of our economy walk out the door. All those third world countries that rely on oil for their food and armies would simply not be important to the developed world any longer. Millions would starve. A power source to replace oil would, simply, destroy our economy over night.
So, how is the next big thing going to destroy the Earth as we know it? The Earth will be fine. The people, however, may find themselves lost before they know it.
What will save the world? Save from what? What is so wrong with what we have that it must be saved?
Poverty (Famine) - The only way to get rid of poverty is to get rid of money.
Pestilence - If it isn't one bug, it 's another. Before HIV, it was Polio. Before that it was TB (still is TB). After we cure AIDS, we will still have Cancer, insanity, flesh eating bacteria..
Space, Quantum, Time - Those are all sound barriers that will fall.
Fear - Without fear, there is nothing left to conquer.

So, Cynthia, did I out do myself?