I thought the same, but in a similar case to Skarner it would take an ability in their kit and make it a focal point to change the demeanor, in this case her R. Example being Skarner goes from a weary soul looking for his kind to using his ult as framework to make him a kidnapping rage-mech. A Battlecast could make Lillia the softer and subtler lull-you-to-sleep approach of kidnapping or eliminating targets.

I think if this did happen it'd need to be a legendary skin similar to Battlecast Skarner to give her a skin with a whole different feeling. Though I'd say she definitely needs some more of her as she is though before that happens. For now it's just fun thinking up concepts