Lissandra's Passive Should Empower Her Abilities Throughout Certain Leveling.
Lissandra is a Witch and a Witch enjoy harvesting power to make themselves stronger. They study books and look for more spells that would make them become more lethal.
So why not keep her current passive that benefits her so very much early game and add a Passive within a passive where she has the power to enhance one of her abilities when she reaches level 6, 9, 12.
In all her skills it will say " Iceborn: "
Her Q's range can be increased and add a 5% bonus slow damage. Her W's root is increased by 0.5 seconds and grants her Armor/MR for a duration. Her E's missile speed is increased and can snare an enemy champion for 1 second ( you'd see the hand holding onto the champion)
This is just a suggestion it doesn't have to be what you read but the idea of empowering her abilities throughout the game just like Viktor is nice.