I like the idea of Chandler and would like to see more of him. I'm not sure he's a good fit for the Grey Council; his time manipulation stuff is really dicey, and if he's caught fraternizing with the likes of Ebenezer, suspicion will be extremely heavy. Aside from that, he's already sympathetic to Doing Things the Right Way, but is Too Valuable to Risk Losing; note that he was pretty much the only Warden on his feet by the end of Changes, and we have no idea how he managed that.
To clarify, I think he's a Grey Council asset, and an ally who can be counted on, but not a member. It's one of those Operational Security things. He can gather and pass information, but actually showing up for an unsanctioned fight is a different story. That said, I'd love it if I was wrong, mostly because I Like His Abuse of Capitalization.
Luccio is a reasonable bet—but only became a candidate for this type of thing post body-switch. I think it's altered her perspective a lot. I know the Grey Council post-dates that, but I think it's an important distinction.
Ramirez is on it. I'm willing to bet a whole lot on that. Though I think Harry would've noticed his green disintegration rays being thrown around.
I desperately want Klaus the Toymaker. He's too freaking awesome.
I don't think the Gatekeeper has time to be on the Grey Council. Maybe in an advise-and-consent capacity.
Maybe Wild Bill? The mad Texan certainly sounds capable, and Harry trusts him to an extent.
Eb trusts Listens-to-Wind and Martha Liberty. Martha in particular is valuable; she's an information-gatherer. We've seen a little of what Listens-to-Wind can do, but not nearly what he's really capable of, I suspect. Not sure he was there at Chichen Itza, but I'd bet you money that Martha provided the coordinates for the lightning-arrival (think about how difficult it would have to be to travel like that without landing on anything).
I think we can eliminate Ancient Mai. She's...too hostile to Harry and Ebenezer. Loyal (to the Council), scary, but definitely not sympathetic enough to break tradition. Same with Arthur Langtry.
As for non-wizard allies:
I'd like it if River Shoulders was involved, but my sense of his characterization is that he's a neutral third-party. He doesn't want to be involved; he just wants to live quietly in peace.
I also don't think the Fae are in on it (though alternate masks, like Vadderung, are obviously possible) simply because they can't lie about it, which makes them a really dumb admission to a secret society. The Erlking isn't on it, but his other mask, Hades, might be (just kidding, that's 100% speculation and I don't have anything at all to back that up; it mostly just appeals to my sense of symmetry for Kringle's counterpart to similarly have a god persona, Hades had a similar reaction to Harry as the Erlking ("you're kinda pissing me off, but I have a certain amount of respect for the size and weight of your cajones"), and I also think it would be cool).
There is also very little chance of Lara being involved; Eb was ready to rip her head off, literally, one book prior. He really, really hates White Court Vampires, and the Raiths in particular; see Margaret's history for reasons why.
Eldest Gruff is interesting, but I still don't think he's involved. He can't lie, he's really tiny, and I think Eb has had it up to here with the Fae in general.
I don't think the Archive can be part of something like the Grey Council. I think it goes against her programming. Though Harry's influence might've changed that. And Eb certainly doesn't trust Kincaid enough.
Denarians are a risky proposition, and while they might provide firepower and intelligence, they're not worth the inevitable betrayal.
I think Marcone might be in on it to some degree. He keeps his word pretty well, and has access to vanilla resources Eb can't really get anywhere else. Plus he has a built-in handler with Harry if it comes down to it. He wasn't at Chicken Pizza, but I think he might be one of those non-combatant allies—or, rather, he provides intel and resources in exchange for things.
How about Eb's contact among the Kenku? Evidently, they owed him enough of a favor to commit hundreds of fighters for a single engagement. That's some serious debt, which implies a level trust, to me. And it would certainly have to be someone relatively high on the totem pole, as they'd need enough pull among the Kenku to provide that much support.
I doubt Ferrovax is involved; Jim's statements about dragons indicate that if one of them got in on things, it wouldn't just be game-changing, it'd be full on game-ending; they're on a scale of power so vast that they need to be measured with a Richter scale (literally—they shift continents).
Beyond that, I don't really know. It's one of the things I really, really want to learn more about in Peace Talks. Though I guess they'll view Harry's Winter Knighthood as a security risk now.