Concept for a More Interesting Lissandra Passive

Most people would agree that Lissandra's passive (as well as Nunu's) are a bit boring. Especially since mana costs and regen are easily subject to change for balance. Lissandra hasn't been seeing much play lately and this could bump her back up.

So here an Idea for her passive:

PERMAFROST: Whenever Lissandra impairs the movement speed of an enemy champion with her other abilities a patch of frost is left on the ground where they were standing (similar to). These frost patches slow (20%?) enemy's who move over them and last (10-20 seconds?).

The duration, slow, and radius of the frost patches could scale with level if need be and easily be subject to balance changes.

Thanks for reading and discuss other awesome ideas below! [slayer-jinx-catface]