This looks like a whole barrel of fun, so I'm purposefully gonna avoid looking at anyone else's answers until I've finished my post. Sorry if I retread over any old ground.
1 ) Harry usually has a few supporting characters in each book. Who do you think will be the main sidekick in Cold Days and in what way will they help? Who else will have a significant contribution to the plot?
This one's pretty much a no brainer. Harry's been removed from his usual companions, and he's gonna have to be navigating Faerie. Toot has lost his source of Pizza payment, and so the 'Za Lord's militia will have largely scattered, but the core members of the Guard will have held together, with Toot as their surprisingly capable commander. In fact, I'd be willing to bet they'll have carved out a nice little domain for themselves among the lower-ranking winter Fae, due both to their history of working together for a common goal, and Toot's surprisingly capable leadership. I wouldn't be surprised if Toot were knee-high by the time Harry starts PT. He'll take it upon himself to show Harry all the ins and outs, giving Harry all kinds of information about things of (seemingly) no pertinence, while at the same time, being nearly useless for the things Harry actually needs. I mean, he may be a bigger deal these days, but the habits formed over the course of centuries as a scatterbrained pixie aren't going to go away overnight. However, something he mentions in passing, something about the relations and politics between the lowest ranking members of the Winter and Summer courts, will prove vital to Harry's eventual solution. I even predict that Harry will realize, either that Summer and Winter are very much in collusion, with both Mab and Titania working together toward a common goal, or that Titania is the one who has been insane all along. Not sure which.
2 ) At the end of Ghost Story, Harry has said goodbye to all his friends in Chicago.
a. Who will be the first friend to find out Harry isn't dead?
Of his 'friends,' Elaine will be the first to find out, or possibly Fix. Elaine will be very much involved in whatever's going down though, and this book will end up either clearing her, pretty much for good, or culminate in her ultimate betrayal. Again, I'm not sure which. Either way, she still has a major debt to clear with Summer, despite what she said at the end of Summer Knight.
Of the Chicago crowd, my money is on Marcone being the first to Know, through Monoc. However, Harry will appear to Murphy first, though she may be in a big-ass scrap with the JLoC when that happens.
b. What shenanigans/snark will Harry use when he "re-appears"?
"I am Harry the White, and I return to you now, at the turn of the tide!"
(Option B: "I got better!")
c. Who will have the coolest reaction?
Butters may well continue any geek quote Harry uses for his re-appearance.
d. Who will have the most touching reaction?
This one will depend on who you're more invested in, as a reader. For me, Molly, who will finally be able to break down and cry, after having had to be strong for so long. The sudden relief will be what nearly breaks her, but the fact that she can still cry will show there is hope. She'll probably kiss him. For many, Murphie's reaction will be the most touching. She'll kiss him too, and then she'll punch him right in the face. Honorable mention, Thomas, again with the fist to the face.
3 ) In nearly every book, Harry usually comes up with a new powerup/tool/way of making things go boom. Given that he just became the new WK, what new abilities will he rely on and which new ability will be the most useful to him during the story (and how)?
Harry will continue to refine his current evocations, as well as playing with more complex combos. I think we'll see Harry use Ice as a defense again, perhaps to make a temporary shield that doesn't require constant juice. Perhaps a defensive igloo to buy him some time to pull off a big working. He may also use it as first aid, to stop bleeding. Extra points, if he uses it on Fix.
I also think he'll learn some tricks about Faerie glamours. Magic disguises of seeming, similar to the potion he used in Fool Moon. He's used them before, and to good effect, but I think he'll get some good ideas here. I also think that we'll really start to see the effects in a couple books when he pulls of a really clever disguise, similar to how he used Luccio's fire needle in Changes. With Fae tutelage and soulfire to give his crafting solidity, it's possible that he could create something that would fool normal detection methods, perhaps even cause difficulty for the Sight (I doubt he could fool it completely, but he might be able to make it difficult to figure).
4 ) Harry generally has a "Big Bad" to worry about. In fact, according to the blurb, Harry is going after someone very specific. Who will that enemy be, and how will Harry take him/her/it down?
I would love it to be Dracula (Baby Vlad, not Daddy Drakul), just for the cheesy dialogue, but I don't think that's likely. We've already got the Fae and the Fomor plotlines going strong. I don't think Mavra is due to show up for another book or two, at least, and it would be disappointing to have Harry make like Buffy without her at least making an appearance.
I'll say Mab will order Harry to kill Santa Clause, mostly because it would be hilarious. I don't think we'll have a clear cut "Big Bad" here, rather several majorly badass proxies.
Still, we need a badguy who really deserves a beatdown. Not really sure on that one. Might come back to this.
5 ) Harry usually faces some sort of "Choice" in every book. A point where the easy way out will cost his soul/humanity in some way. According to the blurb, this book will be no different. What will be the most crucial "choice" Harry makes?
Follow Mab's orders and kill the Clause, or defy her will and face the consequences? Also, how far is he willing to go to preserve Fix and Lilly's lives during their duels?
6 ) As the new Winter Knight, Harry is pretty much garunteed to encounter his Summer counterpart -- Fix. What will their interactions be? Will Fix survive?
Harry will, at various points, have to deal with Fix, the Lily, Gruffs, and a shitload of elves desperate to protect their jolly bearded leige. Fix and Lilly, who are obviously lovers, will attack pre-emptively, and wipe the floor with Harry, but Harry will manage to escape. On the second encounter, they'll take Harry captive, because let's face it, Harry always ends up captive, however, it turns out that they're basically being controlled as a puppet, and Harry will manage to find a way to exploit that to escape. Third encounter, Harry's gotten a hang of the whole knight thing, and he has Fix's number, but he leaves him alive, possibly using that freezing as first aid trick I mentioned earlier. Eldest Gruff will give him a donut, and laugh. Tiny will have thoroughly rustled jimmies.
7 ) Jim has mentioned that it's about time now to start revealing some of the motivations/dealings of the shadowy figures working behind the scenes (i.e. what Harry calls the "Black Council"). What Black Council secrets/actions will be revealed in this book?
8 ) There are many WAG ("Wild A$$ Guess") theories on the boards. Write up to three WAG theories you believe are correct that will be confirmed (or at least have evidence in favor of) in Cold Days.
9 ) Extra credit: Write down any prediction about that you think might improve your final score. Only issues aluded to within Cold Days will be counted.
I'll have to come back to these
