None of us can guess what excites you. That's really for you to decide. You've mentioned a variety of things, some of which are eye candy, others of which are anything but eye candy. A better place to start would be to hunt around SourceForge or whatever for a project that catches your interest and propose to improve it in some way.

For example, you might take one of the empeg projects out there and add new features to it. Maybe you could do something with the microphone input on the empeg, such as an automatic parametric equalizer setting gizmo. Or, if you're less inclined to do audio processing, you could write the glue to make an empeg synchronize with iTunes the same way as an iPod does. Lots of options, and that's only in one small space of ideas.