...a Ghost Rider-like character?  I think that someone, or something, hijacking a Denarian or being in a similar situation to Harry with Lasciel's shadow and using Hellfire for their own motives would be pretty awesome.  Or, heck, even using some Sponsored Magic (maybe Summer Fire?) to accomplish this would work.

I don't have either of the books, so I can't stat a character like this.  Just thought that it was an interesting idea to bring to the community's notice