Taken with the trailer, one of the favors will be to rescue Thomas from the White Council, something Harry would have done in any event, but this would allow him to do with the full power of the Winter Knight Mantle behind him.
I'm pretty much onboard with this idea; however, I'll add some speculation to it and comment on something we saw in the trailer.
Speculating about the one of the favors Lara might have done for Mab, Walter the skull guessed that Lara used to Thomas to kill someone Mab wanted removed and that's what Thomas ends up accused of. I'm not 100% on board with that idea; after all, Mab has Harry if she needs to kill someone, unless it was someone or something Harry would have a problem killing, like someone he cared about. However, I think it's possible Thomas does kill someone on Lara's behalf. Thomas is worried about Justine. If Lara could promise Thomas that she knew how to keep Justine from dying during her pregnancy, that would give Lara quite a bit of leverage to get Thomas to do her bidding. It's also conceivable that Lara knew that Thomas could get caught and she planned to use one of her owed favors to get the Winter Knight's full co-operation and power in retrieving Thomas, if necessary. Harry would do it anyway, but as Conspiracy Theorist speculates, maybe the full power of the Winter Knight is needed to rescue Thomas. Or, maybe Lara likes the idea of the Winter Knight leaving clues behind like a lot of melting frost and ice so she doesn't get blamed for springing Thomas and that way is able to avoid incurring the wrath of the Swartelves.
Looking at the trailer, Warden Ramirez all but accuses Harry of being a traitor. We are meant to think Carlos saw Harry and Lara in flagrante, but what is more likely is that when Harry pays off one the two favors that Lara is owed, Carlos sees whatever it was that Harry did, or he sees a part of it, and starts to distrust Harry.
It's been a long time so it's easy to forget, but Lara owes Murphy a favor. (I think from allowing Lara to quietly dispose of the bodies left behind at the Raith home at the end of Blood Rites.) I have a feeling this is the book where Lara is forced to pay off that debt. It will probably save Harry's bacon.
A couple of people have speculated the fire this a manifestation of Maggies powers. Maybe, but it could also be something as simple as a cooking accident. Bonea has a lot of knowledge but doesn't have the context to fully understand it. She and Maggie get together to make something other than pancakes and boom! One small mystery is who did Harry get to babysit?
Finally, Walter the skull also noticed that Harry is using Demonreach's power or Alfred's type of magic, to infuse his defensive shield. Watch the trailer again and you will notice that after Ebenezer says, "Boy, tell me you ain't dumb enough to try this," when Harry activates his shield, it has pale green color with a slight yellow tint to it. One wonders if this power or type of magic will also help infuse Harry's offensive abilities. To steal a line I saw someone use on Reddit, instead of using Soulfire or Hellfire, Harry's enemies will get Alfired. (OK, it seemed funny the first time I saw it.)