There's more IO needed than the BluePill can handle.  The following user IO needs to be controlled :

• 11 LEDs
• 11 buttons
• 1 switch
• 1 rotary encoder

So we need an IO expander.  The original setup used a MCP23017 to handle all of this.  It turned out to be not such a good idea.  See below.

Other options exist:

1. Use 2 SX1509 boards
2. 1x SX1509 for button inputs + 1x TLC59711 or TLC5940 for LEDs (expensive)
3. GPIO for inputs ( 3 rows + 4 columns). 1 TLC59711 for LED output

16 channel SPI LED driver, non-dimmable : TLC5925 | STP16CPC26 | CAT4016 | MAX6969: 16 outputs, integrated current source, Digikey €1/pce.  Advantage of using these is that charlieplexing is no longer needed.

• SX1509
  • More than just an I²C IO-expander
    • Keypad scan engine on Rows : IO0 - IO7, Columns :  IO8 - IO15
    • PWM dimming engine for GPIOs
  • Sparkfun
    
    • pin spacing : 0.6" x 10pin (2x12p DIP socket could be used : Digikey ED3051-5-ND)
    • I²C connection could be made through 1x5pin socket strip
  • power supply : 1V8, 3V3 (not 5V, but IO is 5V tolerant)
  • LED driving capability
    • 15mA sinking/pin
    • 8mA sourcing/pin
  • QFN package, so breakout board needed.
  • I²C
• MCP23017
  • basic I²C IO-expander, not much extra functionality
  • AliExpress module : €0.76
  • Used by Adafruit
  • 25mA push/pull
  • I²C
  • 2 IRQ pins
  • level triggered interrupts, edge triggered interrupts (no distinction between falling and rising edge interrupts)
  • 16bit
  • 3V3, 5V compatible
  • available in SOIC28
  • This device has no internal keypad scanning engine, unlike the SX1509.  So you'll have to drive the rows & columns yourself when using a matrix keypad.  This causes a lot of I²C interrupts and overhead for the MCU.
  • The IO expander also controls the 15 LEDs in this setup using charlieplexing.  A charlieplexing library for the MCP23017 has been developed, loosely based on the Chaplex library. 
  • Key scanning combined with charlie plexing the LEDs, causes flickering on the LEDs.  Moreover the intensity of the LEDs can't be controlled.

• MCP23S17
  • SPI version of the MCP23017
• PCF8574
  • 8bit

• PCF8575
  • 16bit
  • asymmetrical output driver: 1mA source, 25mA sink

PCB Design

The I²C expansion board can control 8LED/switches.  The LED/switch-PCBs have four LED/switches.  If the I²C expander would be incorporated on the LED/switch PCB, effectively 50% of the I²C expander would be wasted.

All FFC connectors are the same to reduce BoM-cost and lead time.  For the 3V3/I²C-connector, this means that some pins are not really used.