Poison should be SE against water types (poison in ocean = dead fishes).
I agree with hail buff & Ice resisting water
Poison needs a freeze dry esque move. That can hit steel. Acid causes steel to rust.

create an ice type <550BST with abilities we already have that will be viable in OU. No overpowered legendary abilities. I guess you can make an ability up if its balanced.
Hard: make it defensive