Mobile/computer software apps that help people learn to play musical instruments.
Same basic arrangement for any type of musical instrument (mainly Piano, Guitar, Bass, Drums, Singing).
Easy to create. The technology already exists. Would just need to construct the framework, create (or obtain) music files, and create a simple graphic interface.
Could also be expanded to include plug in hardware devices.
It would be easy to create different versions for virtually any type of musical instrument.