AND NOW I KNOW WHAT IT WILL BE - YAY!
Some people might wonder why I was worried about tangible, because emotion can push a plot forward just as successfully, but I have established, I hope, that The Heritage Is Deadly Series is in fact YA paranormal action adventure, and I didn't want to stray from that formula. Yes, Tom has emotions, he's facing an uphill battle to understand what has happened to him, but that was not enough to lay a second book on, nor, indeed to carry the series forward.
Then, I looked back to the stories that Death In The Family is based on, the plots that I've thrown away because I have shifted the focus of the stories. In there, I found a vague reference to an enemy, someone who would have come up in one later story in the originals, a bit of a sledgehammer of a character. However, in that fledgling idea, I think I have found my threat: much more amorphous than in the original plot, an unknown quantity pulling strings, manipulating situations, only slowly being revealed. Book #2 will be this character's introduction, opening up Tom's world from personal things and Coombedown to a bigger stage.
I'm really rather excited about this :D.
Right, must pop off and go plan the plot properly for NaNo now that I have one.