You are mostly incorrect. Ice is incredibly powerful as an attacking type, and needs no more help in that respect; this is mostly true of Grass as well. As such, the only way these two types could truly benefit is for them both to be resistant to Fairy. Giving Ice an attack bonus vs Fairy would not help the usage of the type at all; it would just mean that Water types would continue to carry Ice Beam 24/7.
Poison, on the other hand, is really quite good as a defensive type (barring a glaring weakness to Earthquake), but abysmal for attacking - similar to Steel. As such, it would be most advantageous for Steel and Poison to be SE to Fairy; it helps that this type hierarchy is also quite consistent with existing mythos. --- When life's really getting you down, take solace in the fact that you're going to die.
The thing is though, Steel is such a strong defensive type, it doesn't need to be a better offensive type.
Poison's only redeeming quality is that it is resistant to Fighting, but that doesn't help when it is getting destroyed by Ground and Psychic moves. It is resistant and super effective against types like Grass which are among the worst types.
Ice is a decent attacking type, but again one of its targets is Grass, and it will be less commonly used to counter Dragon with Fairy type introduced. While it does hit some good types super effectively, it is also resisted by the same number of types and is abysmal defensively.
And I disagree about with Grass being a mostly great attacking type. It is only super effective against Water, Ground, and Rock. The first two are great types to hit, but Rock is not used very commonly and is paired with Ground anyways quite frequently. Hitting these three types hard does not make up for being resisted by SEVEN types and being a sub par defensive type.
I think in order to make of for grass type's slightly badness, only or mostly grass types should have health draining moves like giga drain and leech seed. Imo drain punch is one of the dumbest moves ever made, it doesn't even make sense why they thought fighting type would need that, as they are one of the strongest offensive types already. And if they give water type a draining move then idk what to say --- Females are the greatest beings in the universe!!! Users who agree: 16!!!