There are several apps that I'd like to build, if I had the time. This is one:
It's a 1-to-many messaging system, not unlike Twitter, but it only communicates over short range to other phones running the app, possibly by bluetooth or wifi.
When you send a message, it must hop from your phone to another's. It cannot use any other network to propagate. That ridiculous limitation is the essence of the app. IMO it makes it suck, but compelling. People that subscribe to you will get your message if it hops onto their phone. Otherwise, it is just stored, and hops from their phone to any others it contacts running the app. I imagine walking through an airport terminal with the app on, trying to collect what messages other's might be pushing.
It would be and must be ridiculous. If a message is to cross an ocean, it must travel on a phone on a plane or boat.
IMO there's a lot of interesting challenges to building this app.
The one thing that I think it could do that might be semi-useful, is to work as a broadcast network in a protest or disaster.
If by some chance this already exists, point me to it.