An idea to upgrade Lissandra's passive: When Iceborn triggers, make it give her a shield
Iceborn has basically no functionality in mid-to-late game team-fights when Lissandra should have a Rod of Ages completed, thus negating her mana issues. It needs to be given something to upgrade it to make it useful.
My thought is that when you cast a "mana-free" spell, Lissandra should also get a short-lived shield that absorbs damage. This would help her a lot, as she's kinda below-par in durability for a close-range mage. Champions like Ryze and Annie always seem to out-tank her...