I don't think the Caitlin thing is a spoiler. I mean, they have the same name and character design.
As for why she's there: Dawn broke up with Cynthia shortly after DPPt ended, and Cynthia began dating Caitlin.
Because. You know. Cynthia has a thing for lolis.
Anyway, Cynthia and Caitlin had a very, very hot affair that Darach didn't approve of because lolol he is forever alone and jealous of their love. Blinded by envy, Darach devised a plan to break them up -- and it worked. Heartbroken, Caitlin moved to Unova and became an Elite Four member as a means to release her suppressed anger by defeating people with her mighty Psychic-type 'mons.
Present day Unova: Cynthia has arrived to claim her long lost love! Also, unbeknownst to everyone, Giima is secretly Darach, who was so overtaken by guilt he hid out in a cave where he was bitten by Zubats and transformed into a vampire. He became an Elite Four member after ( wrongly ) assuming Caitlin had a preference for them, but the truth is she simply likes older women.
OT ( lol please don't give me the b&hammer ): I don't know if this has been suggested before, but I'd love to see a completely new area in the third game that corresponds to Black City/White Forest. Something that has a bit of both ideas harmoniously married into one. It would probably be something that plays off of the current "Green" trend and run off energy that doesn't harm it's surroundings, like wind and solar energy.
IDK.... This is what I want most in the third game.~