When people try to defend a "Light" type, they usually say that they want an equivalent of "holy" in other RPGs. Something a cleric or a paladin would use. Others counter this saying a paladin would be Fighting/Psychic and a cleric would be Psychic/Normal or something like that. Fighting is the foil to dark, and a "light" type would be redundant.
Only that it isn't really. Fighting can be seen as "honour" and dark as "dishonour". I see a new type being made more along the lines of "compassion", in a different vein completely. This new type would be called "Love" (hey, earthbound did it). And there are definitely spots it can be introduced effectively.
For example, Togekiss is usually said to have some kind of magical "'happiness" properties. However, its typing is the same as pidgey. Blissey has the same properties as granbull, and Audino is as magical as Slaking.
New Love types: Chansey line (Pure Love) Audino (Pure Love) Luvdisc and Alomomola (Water/Love) Togepi line (Togekiss - Flying/Love) Gardevoir - Psychic/Love (learns heal pulse, and its flavour text suits it) Creselia - Psychic/Love Manaphy - Water/Love Eevee would probably get an evo An argument can be made for Clefable, Kangaskhan, Marrowak, Miltank, Skitty, and Arceus (default)
Properties: There wouldn't typically be damaging moves with Love typing. But there would be HP-Love and Love-Forme Arceus would have judgement and so it would need offensive properties.
Super effective against: Dark, Ghost Not very effective against: Ice, Psychic Weak to: Poison, Ice Resists: Dark, Ghost, Psychic, Fire Immune to: Fighting
Love type moves: Heal Pulse Heal Bell Softboiled Wish Healing Wish Helping Hand Entrainment Lunar Dance Heart Swap Sweet Kiss Lovely kiss Bestow After You Follow Me Encore Attract Charm Captivate And the list goes on.