So many players have asked for Light and Sound as new attack types, that I decided to work on the match-up stats. Here's what I have so far; please let me know what needs balancing or changing.
NOTE: I decided to build this type around a literal, physical interpretation of light (to keep religious bias out of the game).
Super-effective against: Dark (well, duh!) and Ghost (spooks hate sunlight)
Weak against: Light itself. plus the following -- Electric (sparks and thunderbolts glow); Fire (flames become brighter and hotter); Ice (semi-transparent and reflective); Steel (most metals are naturally reflective); Water (same reasons as Ice; Light-type moves should work badly underwater, if the official games penalize Fire-type moves there)
Useless against: Grass (plants absorb sunlight)
Regular damage against: All other types
Immune to: Ghost