I was thinking about this during the Falcon 9/Dragon launch, and the lowest-hanging fruit in PDEs probably involves systematizing the wealth of inequalities and quantitative results regarding solutions of PDEs. It probably wouldn't be very flashy, though -- there are a lot of extremely technical results that are only well-understood by a handful of people.

There was an attempt headed by Tao (of course) and others, called DispersiveWiki, but it fell to a spam attack a couple months ago. I'm not sure if it's been rebooted.