I'll try to approach this by the rule of "necessity".
It can't be Julia. Meteor already has a person handling Electric. The same goes for Normal, Grass, Poison and Bug. So, Noel, Shelly, Florinia and Aya should be excluded. They also have Blake, so Ice queen Serra isn't a possibility, and we've already seen Fire in Cal, so I cannot think of Charlotte.
Following the same train of thought, I can't say Samson is a viable choice, nor is Sapphira, because I believe Lin specializes in Dragons already, so having two major figures against us would probably mean that Sapphira would lose her post and somebody else should take the Dragon gym. It could be done with Poison, but with Dragon, it's overkill (and redundant, due to shortage of options).
I can't see Luna joining them. She wants to retire and we've fended off the threat by the name of Bennet, so she's safe.
Plot wise, we shouldn't meet Adrienn yet, but xe was sitting comfortable with Elias and did his bidding without asking much. Xe even followed him when he was setting up our trap against the almighty Ditto. Suspicions, suspicions. (the fairies are supposed to be pure creatures)
Titania is too obvious (then again, if she doesn't join Meteor, she is just a beach)
Now, one of the Circus trio should be working as a Meteor in disguise. Why is there a definitely corrupt officer in front of the Ferris Wheel if not for plot reasons related to accessing Agate? Or, should I say....FeRRRAAAWWWRRRRis Wheel? It must have something to do with Terra's field, as I can't see how her Ground types benefit from the arena inside the Circus (big top). Then again, both Terra and Ciel are newly introduced characters, so it wouldn't make any impact to us if any one of them betrayed us. If you make Shade betray us, we are all doomed.