How about totally removing the slow from her Q and Ult and putting all her slows into her passive.
For example, her passive can apply a stacking mark which lasts for 5 seconds and causes the enemy to be slowed for 5% for 1.5 seconds for each mark on the enemy. There can be a maximum of 10 marks on a single target.
Q applies 1 mark.
W applies 3 marks.
E applies 2 marks.
R applies 5 marks and can "over cap" above 10 marks, up to a grand total of 15.
This fits in with her identity as an ice mage that uses her kit to control the flow of team fights and outmaneuver her opponents rather than someone who just bursts people down. A free spell cast passive is optimally utilized when you don't spec into mana at all, and naturally that means spending all your gold on AP as a glass cannon, since buying CDR won't be practical without high amounts of mana to begin with.
The "ramping threat" paradigm fits in more with her identity imho.