As stated, we know from WoJ that Lasciel is coming back, and she is going to be major league pissed off (the whole woman scorned thing).  IMO, her story has the potential to make Nick’s efforts look like those of just some ordinary school yard bully.

Harry may not have only changed Lash, but in doing so permanently ripped away a part of Lasciel.  We don’t know just how significant the Shadow is to the whole of a Denarian, but my thought is that a major part of the original goes into the creation of a Shadow.  My rationale for this is the connection to hell fire, and the strength needed to provide/maintain that connection.  Think about the pain experience when something is called soul wrenching.  For humans, it’s emotionally painful.  Now imagine the impact on an entity of nothing but soul.  The effect could be both physical and emotional, and incredibly amplified.

I’d love to see not only Lasciel return for revenge, but also to have been driven so insane by Harrys’ actions (the whole unintended consequences thing) that even Nick wants her stopped.  If Lash returns, it probably will not be until the BAT, but I could see Lasciel showing up a book or two before, and setting the whole thing off.