Not sure what to build? Here’s something to get you started.
I love Foursquare and how it helps you get more out of where you are. It has evolved nicely from a broadcasting device to one about discovery and where people are going. While it is still a relatively niche service, it has the potential to grow if it had the right partnership. Like Apple iOS Maps. The Verge proposes an interesting mashup of the two services, where the sum is potentially much greater than the individual parts. I like this thinking and have my fingers crossed for a future combination of the two.