Just saw this on Serebii, this is around the first time when we usually get movie information so it's no surprise, but thought I'd post a thread to remind everyone (and possibly serve as speculation). We'll probably see information in CoroCoro as well.
The trailer seen back in the Summer implied that, like DP and BW, the first movie will focus on the Box Legendaries to some extent. I also imagine that we may see Diancie in it, but she'll probably be kept secret till around February like usual with the Event Pokemon.
I also wonder if they'll perhaps work in the Legendary Birds in some capacity, or perhaps even Mewtwo X, although personally I don't think anyone wants another awkward Mewtwo film at this point.
I'm kind of excited, because XY's been pretty solid so far, and I really want to see what the new crew can do with a film. At the very least I expect it to look nice.