So. If I'm every in a position where I can be an unethically constrained mad scientist type who really wants to make a difference...

I would engineer a virus, maybe delivered by mosquitoes, that inserts the ability to produce the enzyme cellulase into the human gut. This would allow us to eat grass in a pinch, likely solving world hunger. Of course, it would suck to eat grass, so most people wouldn't -- just like most avoid eating dandelions today. But it would be a cool fallback position in a global crisis.