A fun idea for a function to implement: the negative harmony mapping, which is a note-by-note transformation that preserves some character of the note:

  R ⟷ 5 (stable)
  2 ⟷ 4 (unstable)
  3 ⟷ ♭3 (modal)
  7 ⟷ ♭6 (leading)
  6 ⟷♭7 (hollow)
  ♭2 ⟷ ♯4 (uncanny)

 [1] https://www.youtube.com/watch?v=et3CMn2oCsA
 [2] https://www.youtube.com/watch?v=SF8CdxcdJgw