>light attacks are covered by electric fire psychic etc...

I wanna nip this in the bud right away. You cannot say "Dark Type isn't Darkness, it's Evil!" and THEN when folks bring up Light SUDDENLY you pretend we're being literal now. Fuck outta here. Light is just a name, its identity would actually be Holy/Divine/Saintly or whatever, which is a common staple of RPG attacks.

>but fairy represents goodness!

There are different KINDS of goodness. Fairy (when played straight, and not leaning on the grimdark/trickster side of the fence) is good in the sense of an innocent and naive little girl. Compassion, cheeriness, all that lovey-dovey stuff. That =/= the same sort of goodness that Light represents, which unlike Fairy is not puerile.

Fighting Type is actually closer to what Light is, with its focus on law and righteousness. That's fine, I'm not saying there's zero overlap. I'm just pointing out it could work.

Although, despite me typing all that, I actually don't think Light should be a Type—I think it should be a SUBTYPE. It's about fucking time Pokémon's elemental system got an overhaul. Yeah fine I don't really know how it would work (maybe Subtypes do not have typechart interactions) but there are concepts light Light Sound Cosmic Ancient Cyber etc. which do not QUITE make the cut for a Type, but they're perfect for Subtypes.