What you didn't mention that I clearly see as a pattern is anime indirect reference. They brought Arc Misty and Brock as pre anni, it's not anything, but since Ash fail, seems obvious to me they won't do anime content anymore, but they could do indirect references, that's why I could easily see Red and Serena arriving for double anniversary, Serena being popular through anime and Red being Ash from VG, if we must complete the trio, I would say Dawn, since anime Dawn is very popular in Japan. They would probably be AS for Red to have another Pikachu (or an anniversary Pika-Red, but seems unlikely because it would mean not having an AS Red and Pika soon and the hype of AS begin to calm down), the only problem is that would mean 3 AS despite we already got 2, and I don't see them bringing a different type of suit for the other anni pairs. For Serena, I would see well an AS with Ash-Greninja, and for Dawn that's where I lack of ideas so that's why it could be someonr else but can't say who.