Both Mab and Rashid can't be on the Grey Council because one of them has to be available at the Outer Gates when needed. Seeing as Rashid is often unavailable for ordinary Council works he must spend a large amount of time there.
Furthermore, Mab can't be a member of the Grey Council. It's not that her magical strength would probably have allowed her to obliterate any of the Lords of Outer Night, making them rather pointless; though that might be a good argument from a writing perspective. The real reason is Mab had to be available to defend her own realm. In Changes Mab might have had a shiny new Knight in Harry, but while he was rescuing his daughter Winter effectively didn't have a Knight. Mab also loaned out her most powerful vassal; Lea, to Harry, and Maeve being nemfected was at best worthless and at worst a threat to Winter in her own right. Mab needed to keep an eye on her daughter and probably keep her from finding out what was going on. This probably means that Rashid can't be member of the Grey Council either because if Mab is guarding Winter, that leaves Rashid on Gate duty. It doesn't rule him out completely, but I think it makes the Gate Keeper something of a long shot.
I used to believe that because it was stated the LTW became sick he couldn't be on the Grey Council. However, I believe we may discover some of the information we learned from Chandler's letters to Harry is false. (I'm not saying that Chandler is a liar, just some of the information he passed on to Harry was incomplete because Chandler wasn't fully clued-in to what was actually occurring.) LTW may have seen others getting sick and decided it was a good time for him to disappear for a while. There's no way to know this is the case, but it's a possibility I can't completely dismiss.
I won't be surprised if a few of the older Wardens; the ones that don't fully trust Harry, are on the Grey Council. Harry called them hard cases. They probably had a lot in common with Donald Morgan; but didn't have the personal connection to Arthur Langtry that Warden Morgan did. They sure as hell wouldn't have trusted Cristos' peace plan with the vampires. They would have joined the Grey Council because they trusted Eb, and were tired of seeing their numbers diminish with every battle against the Red Court. Even though they didn't fully trust Harry, once Peabody was unmasked as a traitor, a few of the older Wardens may have been impressed with the lengths Harry went to, to exonerate Morgan, even though he didn't succeed. The only older Warden we know by name; other than Captain Luccio, is Warden Thorsen, and he kept Harry from bleeding to death after his fight with Peabody. (That's right in the first sentence of Chapter 49 of Turn Coat.) So I think Thorsen; and maybe one or two other older Wardens, will be on the Grey Council.
What I can't figure out are the "key allies" that Ebenezer mentioned, other than Vadderung of course. The Erl King likes to do his own thing, hunting. There weren't any giant shadow hounds at Chicken Pizza, so that rules him out. Eldest Gruff? I suppose he has the ability, I'm just not seeing him being enough of a free agent to be a member, but maybe after the events of Small Favor he became concerned about the big picture and decided to become involved. How Eldest Gruff would know Ebenezer is another question, Eb has been around long enough for them to have met sometime in the past.