If Listen is still alive, which seems doubtful after Harry buried him under a wall of dirt, he might be a candidate, but I'm just not feeling it.

Listen is a cockroach. Unless the body is seen on page, assume he's going to continue causing trouble.

I doubt he'd be amenable to switching to team Mab, though.

Jim stated that Drakul thought that Kemmler's behavior was amusing, something that was funny.  While that gives us a clue just how twisted Drakul is, I think it also tells us that he didn't rate Kemmler that highly.  Kemmler was a very dangerous person, but even though things like setting up World War One was supposedly one of his projects, I suspect the mad necromancer was somewhat random in his depravity.  Beings like Nemesis and Drakul rank higher on the bad news scale because they have a plan.       

I'm still a bit confused what to make of that bit, given we know from DB that Kemmler's research interests included "how to use necromancy against the black court".  Looking for weaknesses in his elite personal guard seems like the kind of thing Drakul would find pretty far from amusing.

Unless the WOJ meant Drakul found Kemmler's earlier career with the world wars and stuff funny, until he crossed a line in his final publication?  The timing seems awfully coincidental that the Council had been after him for the best part of a century and only got him for good shortly after he published the one thing that might have genuinely pissed Drakul off.  I wonder if Drakul took a page from the Stokerlypse and exploited the council into being his mob to burn down Kemmler and purge (almost) all the copies of his last book.

Also, I keep forgetting to answer the titular question, every time I come into this thread.
I keep getting side-tracked by all y'all's interesting ideas!

Alright, we're all thinking it, I'll say it.

Harry takes Lara to dinner ... at Burger King.  Diplomatic incident ensues.

I think the most likely partner for Summer is the Svartalfs.  Wizards and the Fae don’t really trust one another so I don’t think the White Council would make a good candidate, though I’ve been wrong before.  The Swartalfs probably suspect that Lara Raith had something to do with Thomas Raith’s escape from the dungeon in what was formerly Marcone’s castle and of course they are right about that.  The Svartalfs still have a score to settle with Thomas and they may suspect that Harry has found a magical way to shield Thomas.  The Swaretalfs are normally a very insular people but circumstances might make them rethink their normal policy of remaining aloof.

The svartalfs didn't seem like they held any grudge against Harry for Thomas' extrication at the conference at the end of BG.  Their high lady word-gamed some support for Harry's play to claim the castle from Marcone when she didn't particularly have to.  They're smart, they must have figured out in retrospect that Harry and Lara being out of the conference room when Thomas was taken was a Clue - I'd guess they figured that any help Harry gave was probably owed under an obligation.

Titania and Langtry do have hating Harry's guts in common, though, and Langtry isn't so rigidly honorable as the svartalfs.  That teamup therefore seems like a good way for Jim to torment his protagonist.

Actually, the last time Harry was a detective was in The Law. It's not a super tough mystery, but it is more in line with real life PI work.

Jury Duty was also pretty recent, if we're counting short stories.  Sure, Marcone didn't explicitly hire him to take it as a case ... but he maneuvered Harry onto the jury and counted on Harry being Harry to figure out the case wasn't what it looked like and start his own investigation.

Does it, though?
I think "balance" is one of the critical elements upheld by both Courts.

It's strict for the mantles.  Less so elsewhere.  Like I said in the earlier point, how do you exactly balance Eldest Malk or Eldest Fetch compared to the Eldest of some of the Summer-affiliated Fae subspecies?  They have inherently different powers and roles in their nations.

I think the reason a little more flexibility is allowed other than in the mantled roles is that Winter just allocates its overall surplus to the Gates.  Even if there isn't a 1:1 counterpart for every single role, they can maintain an aggregate balance with Summer by shuffling forces between Gate duty and the home guard.  Letting Lea be strong enough to cover for her in shifts at the Gates is a big part of why Mab allows her to be as strong as she is.

• As you said KSG, The Librum Bellum aka Men in Black aka the Librarians presence will be there, probably subtly. I suspect we may even meet one of their agents. A new, highly dangerous threat. Even Vadderung was wary of them. A lot of what Harry does in the book (hopefully helping people) will be noticed, but also some of the more destructive stuff. ... Was it one of their agents in Dog men?

That's a question that's bothered me for a while.  Because the Librarians are supposed to be highly secretive, and particularly cautious of Harry in part because he publicly advertises, and also because of his propensity for destruction.  I recall a WoJ that the main reason the Librarians never reached out to Karrin was that she was considered suspicious for being a known associate of the wizard.

The feds in the Dog Men comic weren't exactly enthusiastic to cooperate with Harry, but they weren't super shy of letting him know they were aware of the supernatural in general and specifically of him, either.  It's been a while since I read that one, but I don't think they came across as being as impressively prepared / capable as I'd have expected from the Librarians, either.

So are there two branches of clued feds?  The Librarians in the background being super cautious, and the more goonish branch doing more open field work?  Either independently, or directed from the shadows by the Librarians, as pawns that known just enough to do their missions, but not enough to compromise the Librarians if something happens to them?

I wouldn't be surprised if they try to get Tilly working for them, but probably not trusted at too high a level, as making their Chicago presence someone who more or less already trusted Harry would make it a bit too easy for him.