I've played this game before--and I will again. I find it clears the mind wonderfully to wonder what you'd do for the world if you had a billion dollars to spend. Build a secret volcanic island lair? Check. Cure necrotizing phlombosis? Check. Oh, there's all kinds of stuff you could do. 

--There's one rule, though: whatever you spend your billion on, it has to be something nobody else is doing--and something that's worthwhile in a completely game-changing way.

After all, in today's market a billion dollars will get you a few miles of subway, or a new sports stadium. Yay. But it can get you so much more, as Elon Musk has demonstrated with his reinvention of the space launch business (and he hasn't spent more than a fifth of a billion on that). In fact, a billion is enough to solve more than one problem, if it's properly distributed. 

I play this game regularly because the world keeps changing, and what's important keeps changing. Some items remain from previous lists; some are new. Here's today's list:

1. $200 million to studying and developing new systems of governance. --No, I don't mean e-voting, or even e-democracy. I'm talking about a systematic study of how humans govern themselves, and how our cognitive biases and interactions at different scales scuttle effective problem-solving among groups. Think this is fringe science? I happen to think it's the most important problem in the world, the only one that counts. Because if we reinvented governance (on the level of individual self-control and choice, on the level of small-group interactions, and all the way up to how millions of people make collective decisions) then every other problem facing us now would become tractable. So I'd be exploring cognitive science, promise theory, structured dialogic design and a lot else besides.  $200 is really far too little to spend on this, but it's a start.
2. $200 million to develop efficient and economical carbon air capture and sequestration. Carbon air capture is the only potentially feasible method of returning Earth's atmospheric CO2 balance to pre-industrial levels in less than a hundred years. Emissions controls won't do it, neither will renewable energy, or even the complete disappearance of human civilization. The CO2's there. It has to actually be removed from the atmosphere. Currently, far less than $1 million is spent per year on how to do this. And that's just crazy.
3. $200 million to develop a microwave space launch system. --Again, this sounds wacky. But the physical resources of the solar system are effectively infinite; and the world looks like a very different place if you play the game of imagining that access to space was really cheap. All sorts of currently impossible problems fall like dominoes if it costs as little to get to space as it does to fly across the Atlantic. And, in space development, there is only one problem, and that's the cost of going the first 100 miles. Literally every other issue becomes tractable if you solve that one. So let's stop dicking around with incredibly expensive launch systems and solve it.  (Why microwave launch and not laser launch? Because microwaves are more energy efficient, and can be done now; and because I think laser launch is a political non-starter, because accidental or deliberate straying of a laser launch beam could blind or fry anything in the sky, including airliners or other nations' satellites.)
4. $200 million to finally realize the dream of nuclear fusion energy. We are that close. Most of the money would be divided up between the chronically-underfunded research projects that are getting close: IEC fusion, magnetized-target fusion, and several others. I'd fund General Fusion's steampunk pneumatic-fusion system, for instance. But I'd also fund one method that nobody's trying right now, but may be the best of all: levitating dipole fusion. 
5. $200 million to prototype the business models, supply chains and build a first-generation Vertical Farm. Because sane governance, free energy, a solution to global warming and unlimited material resources aren't enough if half the planet's starving, which will be the case in forty years if we don't act now. This one seems like a no-brainer, if it can be properly optimized.

An odd set of priorities? But, what if they all worked? Simultaneous breakthroughs in energy, resource access including food, removal of the threat of global warming, remediation of the natural environment destroyed by intensive agrivulture and, most importantly, a Renaissance in collective problem-solving would literally mean the world to us. 

The point of all this should be clear. Even in a global recession, money's not the scarce commodity. Audacity is. 

What can you do with a billion dollars? 

You can build a new sports stadium.

Or, maybe, you can save the world.