Different kind of musical k/b
Instead of just one row of boring old keys, have multiple rows of keys - but not like the k/b's that already have multiple rows: each key would merely be a square, and the rows merely comprise a matrix of squares - a lot simpler. If it's a midi k/b then, of course, extra rows can be used for anything, such as other instruments, sound modifiers, or different octaves.
The question is still raised of how to handle black vs. white keys. One way is simply to have black columns in the matrix which are thinner than the white columns. Another way is to forgo the black vs. white distinction altogether and simply have 12 equally positioned keys per octave.
While we're on the topic, an even more advanced k/b would just have a big touchscreen and imagination would be the limit as to how it might interface with the user.