I think it would be quite an interesting project to start an RC Plane Autopilot project. It is actually my number 1 project in mind. I'm currently using ArduPilot in my planes/quads, so I think it will be fun to play around with the board in the air. With the event driven nature of espruino, I think its a perfect fit.
Distance sensor (HC-SR04)
Better MPU (MPU-6050?)
PPM signal generator/reader (found a project using arduino, http://code.google.com/p/read-any-ppm/downloads/list and http://code.google.com/p/generate-ppm-signal/downloads/list)
Possible to control an on screen display overlay right from the board with onboard camera? I know the OSDs I have seen are dedicated AVRs, so may not be feasible?
If you can't tell I'm a software guy by trade, still green with some of the low bits of hardware :)