I too agree with the idea of a keyboard piece (made like a 1 x n tile only with raised detail instead of just printing). Done right, it could be used for a keyboard, piano, pipe organ or otherwise.
Needless to say, such a part should have appropriate detailing on the black keys.
Thought I just posted on this, guess I did something wrong.
I registered specifically to ask for such a piece (pieces). A plate style Lego piece with raised black keys (in sets of two and three) with etched white keys in between.
Perhaps each brick should be just one octave so that you have the option of making larger (grand piano) and smaller (accordion) instruments.
This way you can simply add bricks as needed to create keyboard instruments.
For the sake of those that wish to make a "keytar" you -could- also make another plate just for that (where a single octave snaps onto it).
Of course these should be in scale with the minifigures.