Imo, it’s not an issue for sets to boost most, if not all, skills. Boosting specific skills is where legendary powers should come in.
Random legendary powers:
• Enemies hit by blizzard takes XX% more damage from your pets, blizzard deals XXX% more damage and last X seconds longer.
• Casting a blizzard over a hydra will destroy the hydra and release a wave or nova that deals XXXX%.
• Increase Mirror Images life and attack speed by 50%, Mirror Images deal 100% more damage.
• Upon taking fatal damage, instead kill an active Mirror Image and heal to 70% of your maximum hp. This effect cannot occur again within 125 seconds.
• Hydras within 30 yards of each other are linked together with a fire chain that deals XXX% damage. Regenerate 1-3% of your maximum arcane power whenever a linked hydra attacks.
• Casting three blizzards within close proximity of one another, turns the blizzards into a slab of ice that’ll deal XXXX% over 3 seconds. Or Blizzard now deals XXXX% over 3 seconds, but now cost 120 arcane power to cast.