if there was a new type, what would it be and what types would it be effective/ineffective against?

To be honest, I've always thought that a light type would be really cool, but I thought there would have to be a MULTITUDE of new light pokemon to balance things out. It'd probably be good against darkness, but also would be weak to it too, kind of like how dragon is super effective on dragon types. But I also realized that poison type doesn't have very many things that it is super effective on, grass type primarily, so maybe there could be a new type that poison could be effective on?

I think they should bring back ??? and make it an actual type, since in the gen 4 games there's already a ??? arceus sprite, and I think it should be neutral to everything and do neutral damage to everything.

yea! and the moves would be very powerful, or have special attributes such as setting up a light screen right after attacking.

I like the idea of light. Maybe these could be its traits.
Weak to: Poison, Dark, Psychic
Resistant to: Steel, Dragon, Fighting
Immune to: Ghost

Super Effective against: Dark, Water, Ghost
Bad against: Ground, Ice, Psychic
Useless against: Grass

Yea, those are some realistic type matchups! I suppose it is always a possibility, yet I don't know what nintendo intends for its audience in terms of the future of pokemon. Nobunaga's ambition looks quite hit or miss depending on the audience XD.

*miss*