I had a few related to electric guitar and bass, but each time it turned out that they already exist.
One idea I had was to make an electric bow for bass and guitar which uses magnets to vibrate the strings. It already exists, it's called the eBow.
Another idea I had was to create pickups and potentiometers that use quick disconnects instead of solder joints, so you can have a completely solderless system. It turns out EMG has been doing that for years.
I thought of designing guitar tuners that tune automatically, but Gibson does it already.
I'm going to have an idea one day that nobody has thought of

one day...