I assume that at this rate, sometime this decade a vendor will start selling switches that each have a little speaker in them to make them sound different
other ideas:
- haptic feedback instead of mechanical feedback
- electro magnetic "springs" so you can adjust how heavy switches are on the fly
- tiny little bell at the bottom so it rings when you bottom out
- mineral oil submerged keyboard
Other idea, a rotating dial that is plays a slide whistle as you rotate it.