Info about the passive and item interactions: As far as I can tell, the empowered auto attack has 100% ap scaling, 100% Iceborn (passive) scaling, and 100% ad scaling. It gives about 6 seconds to use the empowered auto. It DOES NOT convert physical on hit effects like bloodrazor or blade of the ruined king to magic damage. It does convert the damage from Triforce and Iceborn Gauntlet to magic damage. It does not convert the extra bolts from Runaan's Hurricane to magic damage. The empowered auto also can crit and is effected by Infinity Edge. Does not apply effects like Liandry's Torment or Rylai's Crystal Scepter.
In laning: due to her relative short range, high mana costs, and lack of health sustain, this is one of the more interactive stacking passives (in comparison to Nasus and Veigar). Trying to get two stacks from each ability cast safely (1 from ability, 1 from empowered auto) felt like nice way to encourage a more tactful approach than just q spamming.
Mid Game: It's nice for turret taking and it synergizes well with Lich Bane (not sure if either were intended). Adds a lot of damage to her kit, but that may also be the more damage build being encouraged.
Summary: Unlike her old passive, which one mostly just forgets about except once or twice a game, this feels like a passive she can play around. The mana scaling might be on the weak side, but overall this is a huge boost to her satisfaction while playing. These changes, however, may push her out of midlane primarily and into top.
PS: while work is being done on Lissandra, have you thought about changing her q indicator to being like Grave's Ult and making the ability turn into shards at the end of the range even if it doesn't hit an enemy? Not saying it should be done, just wondering if you guys have looked into it.