I'd cross out Shelly. We already have an E4 Bug trainer that turned evil. Giving them 2 doesn't make any sense. Thinking type-wise, we have Sigmund as the evil electric boss, Sirius for some poison, Fern for grass, so I would not expect Julia, Flobot or Aya to join. That leaves me with Radomus and Titania. The latter wouldn't surprise me much, as we've seen her secret meeting at the woods and we know she does have some connection. Radomus seems nice but he knows too much and reveals too little. He could be a sad father, who adopts Luna at the loss of his real daughter due to the meteors (sidestory: Anna is his daughter, but she was kidnapped at early age, he was asked for ransom, they told him she died, so she would not have any legitimate parents and give her to Sigmund) Serra is a viable choice. Either she joins to protect her son, or she is promised the beautiful world in which she's relevant again. The others: Shade: I don't really think that he really has any reason to join them. Usually Ghosts are neutral and care only for themselves, neither good nor evil. Otherwise, he is a pretty nice character, since he helps Anna. It wouldn't make any sense for him to join. Agate trio: Samson didn't even know Kiki died. No connection to the meteors. Ciel lives for the circus and Terra, oh Terra, she is quite crazy, so even if she wanted to join, they might not let her.
Noel: Too cute and innocent to be the evil mastermind behind the whole sceme, who is the real reason behind the burning of the Tanzan cove house, and conspired with Sigmund to get all the children into custody, while appearing to be one of them, yet never participated in any experiment. Noel doesn't play with your rules, HE CREAtES TEHM!!! He uses Clefable in a normal team, and that tells you all you need to know about him. Hail Seadra!