Do you find the idea of dashboards for businesses interesting? That's what I'd work on if i had lots of time to research, lots of know-how, and lots of business experience. It's exciting because

a) building these things makes the world more productive, businesses more agile,

b) you can take inspiration from a wide range of sources (science fiction, journals, movies, video games, old inventions),

c) there's very exciting stuff coming to market right now. Microsoft's Kinect, Nintendo Wii (and whatever their new console will be), lots of interest in 3D displays, while stuff like voice-recognition and natural language interfaces and data visualization is finally getting really good

So many combinations of the above to be discovered, and so many industries to benefit.