I used to be moderately on board with the idea of more types, but now I'd rather they just add more move classifications instead. By "classification", I mean how there's a whole group of Powder moves, or Ball and Bomb moves, and Pokémon types and abilities interact with these sorts of moves in different ways. I don't know if there's an official term for it, so I'll just keep using classification for now.
So just as a whole bunch of moves are classified as Sound moves rather than having a Sound type, other commonly requested new Types could become Classifications instead.
Wind would be a good one. A lot of Flying type moves would fall into this, but so would Fairy Wind, Silver Wind, and Twister, among others. Wind attacks could gain a slight boost when Tailwind is active, and could also gain priority from Gale Wings.
Light is another new Type which a Classification could do just as well. Solar Beam, Mirror Shot, Flash, and other moves would be classified as Light. A new hold item, Magnifying Glass, could add a chance to Burn to all moves in the Light Classification.
And I'm sure there are other new Types that could receive the same treatment.
(Wait, is this too far off topic? Should I spin it off into its own thread?)
