add option to play the song in the transposed key with simple chordal accompaniment with tempo control
singers could find the key thats right for them if they could play the transposed version. they could experiment with various transpositions before printing out.
I would suggest a vocal Aah timbre or organ flute tones and very subdued chordal bacgkround with acoustic bass on the first beat or with chord movement. It would also help singers to hear the original melody rather than relying on the versions of the great artists.
features like tempo control , pause, rewind, fast forward would help singers navigate through the song and study isolated sections.
an octave shifter would be very useful as its easier to sing along to a line when it is in your octave.