If they were to change Ice type for Gen 7 as a defensive typing like they did with Steel in Gen 6, how would they go on about and fix this horrible typing?

The same thing may apply to the Rock typing, but Rock has much more resist that Ice, it just has weaknesses to so many common attacks.

>Ice
>Resists: Ice
>Weakness: Fighting, Fire, Rock, Steel

>Rock
>Resists: Fire, Flying, Normal, Poison
>Weakness: Water, Grass, Ground, Steel, Fighting