My modest contribution to this rich space is to enhance the illuminated button arrays (in this case from Sparkfun) with pressure sensitive resistive fabric [2].
I use a single "Teensy" 2.0 microcontroller to do all the led driving and key multiplexing. Sparkfun uses five chips for this where I only used one and theirs unfortunately is not wired in a way that would be easy to adapt to read pressure values from the keys.
I shared this design with Keith McMillen who went on to make piezoresistive pressure sensing products such as the Soft Step and Quneo.
I shared this design with Novation during a visit to High Wycombe to encourage them to add pressure sensitivity to their button array product (LaunchPad). They later released the LaunchPad Pro which does have it although I don't know whether they use oen of the alternatives to fabric: piezoresistive ink or molded conductive elastomers.
Links:
[1] http://en.wikipedia.org/wiki/Salvatore_Martirano
[2] https://www.adafruit.com/product/3669
[3] http://www.adrianfreed.com/taxonomy/term/173
[4] http://www.adrianfreed.com/category/affinity-group/e-textiles
[5] http://www.adrianfreed.com/taxonomy/term/183
[6] http://www.adrianfreed.com/taxonomy/term/174