People keep saying Adrienn, but I doubt it. Xe seems more like someone who would take more a sensible approach to restoring the region. Perhaps if Lin was not on charge of Meteor, he would join, but as it stands, it is unlikely. If Solaris were still in charge, could see Xim joining them, but Lin's methods are too dark and drastic for the Fairy Gym Leader to condone.
Radomus is a genuinely a good guy. Sure. he may be a bit quirky, but in the end, he wants what's best for Luna and G.G. He may be a master of deception, but I feel if he were to join the Meteors, it would have happened back by 7th Street, when the Meteors stormed the castle and would have provided pretty much a free pass to retrieve Luna.
Aya betraying because she's angry at Cain? No. That's not happening. Let me explain siblings to you. If you have them, you hate them. You hate them A LOT. And yet, you love them and will always be there for them. Plus, if you chatted with Cain and Aya while they were at the circus, you would see that after some banter/argument, they make up and are just happy that each other are okay. Also, as calm as Cain may be right now, he's probably tearing himself up on the inside for not hanging back with Aya and protecting her, as annoying as she would have found it. Aya is probably furious at him at this point, but she wouldn't betray over it, if anything, she's probably biting the hands of the people who are trying to feed her right now.
Hardy seems like the most likely option, since he's desperate to retrieve Aya. He doesn't seem to grasp that the second that the Meteors have what they want, they'll destroy the city. He might whip out his level 90 pokes or something and steal the jewelry from us to save Aya and we'll have to go stop him or something.