Story

What would you spend a billion on if:

1. You had to use it all on a single thing; a science or technology or engineering improvement not directly benefiting yourself, family members, etc. any more than any other random bystander.
2. The billion has to be enough to either make or break it, i.e. either the aims are realized or they are proven not to be viable for the near future.
3. The choice with the combined highest likelihood of success and most benefit gets the billion, i.e. a smaller likelihood of success can be outweighed by massive benefits or smaller benefits can be outweighed by near certain likelihood of success.

No, I don't have a billion :( but somebody must have xD
Considering the amount of money flowing around some country or individual (or one billion people contributing) could easily make it into an annual award.

I don't know what the list of choices should be (or how the debate will turn out) but I'd like to suggest Focus Fusion/Lawrenceville Plasma Physics as an alternative.