A type of episode they should have done.
A Christmas episode.
The premise is that the dead person who was killed years ago was killed wearing a Santa suit. He had no wallet and no ID, so he was never identified.
Years later, DNA discovers the identity of the dead Santa, so the case is reopened, and Lily investigates.
It could be set at Christmas, and the music used could all be Christmas songs. The episode can still maintain its dark edge, but also have some lightness and humor in a couple of scenes.
It could also have Lily dealing with Christmas, and realising that she is a bit of a loner, and tries to deal with that.