Light-type, simply to counteract the Dark-type. Moves like Luster Purge, Flash Cannon, and Doom Desire (to an extent) bother me due to their blatant references to LIGHT, yet the former is Psychic-type, and the latter two are Steel-type. I know that it would be kind of hard to make a special Steel-type move, but still...
There should also be revised type match-ups, like Fighting-type being super effective to Bug-type (think "fist smashing ants/roaches"), Poison-type being super effective to Water-type (there's a reason many organisms can't thrive in
polluted water), and Electric-type being super effective against Generation 2 (really, metal CONDUCTS electricity. This should have been implemented in Generation
2). If Ice-type can't have super effectiveness against Water-type, at least let it deal neutral damage (I mean, all of those "Ice Rays/Beams" in videogames being used on Water...?).
The Light-type and Dark-type would be mutually super-effective against each other... I kind of want to make it super effective against Flying-type (the "Icarus" tale; flying organisms being more prone to the Sun's glaring light), but Flying-types already struggles enough in battle. Light-type would also be super effective against Steel-types. It would be weak to the aforementioned Dark-type defensively and offensively weak against the Water-type and Ice-type (derived from the concept of light
refraction in water and ice crystals). Ghost-types could be warded off by light, I guess. Fire and electricity can be construed as manifestations of light, so there's that... And Grass-types commonly use sunlight for energy, so they'd probably resist it...
Light-Type Matchups:
MOVES:
Super Effective Against: Dark-type, Steel-type, Flying-type, Ghost-type
Not Very Effective Against: Water-type ,Ice-type, Fire-type, Electric-type, Grass-type
DEFENDING POKEMON:
Susceptible To: Dark-type
Resistant Against: Steel-type
Now that I rethink it, lasers could theoretically
slice through ice...so I guess 1x damage to Ice-types. Now the number of offensive and defensive resistances and weaknesses are equal. This was what I was able to scramble at the moment; I don't even know if they would be viable and/or possibly break the metagame even
further. :\
But yeah, after the Light-types inclusion, I would vouch for NO MORE TYPES. The only one I could even consider conceivable after that is a "Crystal-type" (derived from the
Crystal Style/Crystal Release of the
Naruto series), and I'm a bit averse to that as Ice-type seems to cover that "transparent light-refracting solid" spectrum, and we already have THREE "mineral" types in Pokemon already.