Poison imo should not only have a resistance to fighting, but dish out 2x damage to fighting types, since poison prevents connections to the nerves and muscles. Poison is also, in a nutshell, unhealthy...and you have to be healthy to train in the martial arts and such. I think it would also be interesting if poison were super effective against psychic types because of how it would attack the brain. Now correct me if I'm wrong, but that would be the first two types that are super effective against one another (although I myself do not really understand why psychic is strong against poison).
I don't really go for the "pollution" aspects of poison people are talking about. You can pollute the water, you can pollute the air, you can pollute the earth...and then you would just say that poison is unhealthy for all living creatures so pretty much poison should be super effective against all types (except for poison and steel). Meh...
I really want bug types to have a resistance to psychic and dark types. Not for any particular reason based on what they mean irl, but because bug types need more resistances. Imo, they should have complete advantages over psychic and dark types.
I also still think that water should be super effective against steel. Yes, rust. I also think electricity should either be super effective against steel or steel should be immune to electric types. Metal is one of the best conductors, so either the electricity would easily pass through the metal and into the pokemon's inner weak spots to inflict a lot of damage, or it would just go straight through the pokemon altogether, which is why I believe it could be either super effective or not effective at all.