a few simple suggestions

riot give the old passive it would work great with her play style since it revolves around getting into very close range to CC and burst a target. with the old Xerath passive she can have just a little more safety as it gives armor equal to a % of AP. right now lissandra is pretty low range for a mage and her burst isn't very high compared to a lot of longer ranged mages like but i accept that being as lissandra has a kit filled with decent CC and lock down compared to them.the trouble with current lissandra is her passive is borderline worthless late game and mid game. not only that but it forces her to have somewhat larger mana cost to compensate for the free spell and it makes very little sense to anyway.my other suggestion would be to make a completly different passive called "Arctic presence" in a area around her she has a blizzard which deal a small amount of % max health as magic damage persecond like a null zone that follows her around it could also applies small to moderately strong slow to champions in it that intermediate stops after they leave the zone. lissandra's whole thing is that she has to get into close range to get her spells off and cc as many people as she can before she dies and right now her passive just isn't much of a help in this regard. the reason i made this is riot most likely will be updating mages similar to marksmen soon or at least in time.