Missing:

• Famicom Disk System
  -- add L and R to nes.cfg (think in a button like "change side A-->B" and "change disk" for example), https://github.com/libretro/nestopia/blob/master/libretro/libretro.cpp#L354
• Nintendo 64
  -- an overlay with d-pad, start, A, B, c-buttons, L and R
  -- other with analog, start, A, B, c-buttons, R and Z
  -- other with d-pad, analog, start, L and Z
• Jaguar - too many buttons... one overlay to d-pad, pause, option, A, B, C, and other to the 9 phone buttons? Not a priority, virtualjaguar will not run smoothly on mobile anyway...
• Sega Saturn
  -- a genesis 6-button overlay like, with L and R
  -- another with analog
• WonderSwan
• Lynx
• Atari 2600
• MSX / MSX2 - d-pad and two face buttons? It seems there's no standard...
• Dreamcast - easy, retropad-fast with flipped face buttons
• Vectrex
• DOOM? Quake? Cave Story?
• Arcade overlay using D-pad scheme from wonderswan (different approach in diagonal) & analog stick
• Make a keyboard overlay too?