Bold speculation time. Hoping these haven't already been said in the thread.
Starters are Grass/Dark, Fire/Psychic, and Water/Fighting
Look at :36, :44, and :52, respectively. They each use moves that seem like they would be the aforementioned types. In addition, having this sort of triality would mean the Rock-Paper-Scissors mechanic of the starters would work both ways.
The Legendaries are Dark/Flying and Something/Fighting-type, and are Trio Masters of Gen. V
Dark/Flying should be a given for Y's mascot. Generation V has shown us that not all Fighting-types necessarily have to be bipedal to have their typing, as seen with the Muskedeers. Among that thought, X's mascot looks very Deer-like, something that would fit in with the other Muskedeers if it was smaller. Additionally, every even-numbered Generation brings about a pair of trio masters, so it wouldn't be too far off to think this pair is part of the pattern. As such, Y-Bird, whom is similar to Tornadus' Therian forme as pointed out, could very well be the therian form of yet another Kami. Likewise, X-Deer could have relevance to the Muskedeers, leading to a likely type like Ice/Fighting or Psychic/Fighting.
The themes of Generation VI are Chaos and Order
Frankly, with Yin-Yang out of the way, there really isn't anything left but straight-up Good and Evil/Light and Dark. Dark is known to be a chaotic type, which no doubt Y-Bird will be, and X-Deer definitely has that regal appearance to look like a lawful deity (that lawfulness tying into the Muskedeer theory once again). Hence, the games will go into the evils of both
Chaos and
Order, and how the both require a balance... Cue a pair of new villainous teams.