Some people have been saying that Psychic and Fighting is identical to Light, but in my opinion... it doesn't feel like that unfortunately. 'Cause I feel that Psychic-type was based on "Kinetic Energy Power", which has nothing to do with Light at all. Thinking of what the move Psybeam is about... it seems that it's about using some kinetic energy beam at the opponent, which again... kinetic energy doesn't mean that it's based on Light. Same goes for Fighting-type in how it is unable to effect Ghost-types, "Fighting-type" comes from real Fighting, not as in "Fighting for Good"... it's about actual Fighting, so it can either be good or bad, which again I feel has nothing to do with Light.
Either way, the only moves that would enable other moves to be Super Effective against a Ghost/Dark-type in Generation IV and onwards, is to either use the moves Foresight/Odor Sleuth (to remove the immunity for Ghost-types have against Normal or Fighting types) or Miracle Eye (to remove the Dark-types' immunity to Psychic-types, then use the move that is Super Effective respectively, but it still costs two turns to do so. :P
Unless there is a new type to counter the immunity that Ghost/Dark-types have, probably not gonna be possible to take down a Ghost/Dark Pokémon in one hit unless the Pokémon is at a higher level than the opponent's Pokémon to do so, or uses a powerful enough move... which at the same time, would indeed have to be either near the opponent's Pokémon level or higher.
But having said that, the Dark/Ghost-type does indeed show that the type chart is still unbalanced. Some of us might be thinking that a new type may make the type chart unbalanced, but I believe that if they introduce one Light-type it may actually fix the type chart. I would recommend having a think about it and reading up about the types before making your final decision about whenever there should be a new type or not. Since we all have our own opinions, and that's fine, I respect them. ^^
As far as I know, Psychic-type was immune to Ghost-type moves in Generation I, but this was fixed in Generation II when they made Psychic-type weak against Ghost-type moves, which kind of balanced it out a bit. The Dark and Steel types were more to balance out the over-powerful Psychic-type and the underused Fighting-type. But this does kind of raise the question of "How would it have been if there wasn't a Dark-type or Steel-type, but had the same weaknesses and resistances as Generation II?". It quite possibly would've been interesting to know what it could've been like. xD
But if Light-type was introduced, I would be expecting weaknesses and resistances to be somewhat more like this:
Weak against: Fighting ('cause anything fighting can be good or bad), Grass (because plants photosynthesize light) and Normal ('cause anything Normal takes in sunlight, but can harmed in some way due to the UV, as opposed to other types)
Strong against: Dark (light lights up a dark area), Ghost (don't like places with light) and Dragon (most live in caverns, not used to light)
Immune against: Dark (because dark energy can't blot out light)
Can't damage: Fire ('cause light can't do anything to a fire)
Giving Normal-type a type that it would be strong against would surely make things more balanced. Since Normal-type doesn't even have a type that it's strong against yet.
I see it as a definite possibility. They've done it before. What's stopping them from doing it again?
Light, Cosmic and Sound types appeal to me.
I think Cosmic-type is pretty much Psychic-type as it is. 'Cause it still represents the same thing: kinetic energy! :P