Light types should have been come in Gen 6 when they had the chance. Fairy pretty much now covers this. Attacks like "Dazzling Gleam" "Moonblast" etc, etc, etc, are all light based, yet are typed as Fairy. So making it a seperate type would require GF to change the types of these attacks, or of any light-based attack, such as Flash Cannon, to be light.
In fact, instead of people thinking there could or should be a "Light type", I might have a better explanation as to why there's no Light type specifically:
How does the theory/idea stand that perhaps all Pokémon are light by nature, as in via their life-force, which explains their ways of evolving: always glowing radiant bright when they evolve, etc, etc, and therefore there is no Light-type specifically, because light is the inner spiritual life-force energy of all Pokémon by nature. Driven by nature's light/radiant energy.
That is how I would deal with the absence of a "Light" type in my fantasy. This is actually made stronger with Aura, which is depicted in the Lucario movie; being described as the radiant life-force of all living things, otherwise in both human and Pokémon, hence why there's no Light-type. Not because it can't be done, but because its already there, within all Pokémon by nature.
Its like the Spectrobes, which are described as "creatures of light". The same could be associated with Pokémon, and perhaps even the lore of Mega Evolution via its rainbow symbol, the Sun Dial, etc, etc.
I could even dare to say, perhaps, that, if we go with this idea, that people should consider that the Light-type is likely more of a primordial force that drives all Pokémon. In essence, the power of the Light "type" resides within the spectrum of all 18 Pokémon types in some way or another. Perhaps this might be a focus even in the Gen 7 games through its regional professor or some other source. They're after all named "Sun" and "Moon", objects that give off natural light.