I play a lot of Lissandra, and I've recently been thinking about this myself; I don't think she needs a new passive. I think the problem is that it feels underwhelming due to lack of thematic tie and poor visual feed-back (compare Liss's free spell-casts, to Xerath getting a huge chunk of mana back with a nice visual and SFX, Xerath passive 'feels' better).
I mean there is even less going on with Xerath's passive but people don't seem to mind. Lissandra's passive ties in to her gameplay and is interesting in that way (using your spells optimally gets more efficient mana saving). It's a 'true passive', and not a psuedo-passive gimmick like some recent ones. I think it's okay right now.