After two years, I'd expect to see... an old new Unova in the limelight again. I probably will also expect to see yet another pair of new protagonists, and the starters will be reused from Generation V again - you can probably see the games as something that straddles between Generation V and VI, in part by providing a closure on the Unova "trilogy", and also in part by providing direct continuity from the Generation V games to Generation VI in general, and with all the Pokémon being very familiar to anyone who had played BW or B2W2, it can be considered an "old new" game. Old, in the sense of keeping the Generation V spirit, and new, in the sense that it is a new game which can be treated as stand-alone, and also be able to relate to Generation V and VI directly, at least in story and continuity.
The games will be more story-centric than any other Pokémon game ever released, including Mystery Dungeon games and previous Unova games. In particular, I'd expect the story to focus a lot more on N himself and "What happens when you put all three of them together?". There will be something in the games that causes a third, unique, Unova-specific legendary Pokémon that will be in the Generation VI Pokédex proper, with said Pokémon back-ported to XY via 3DS patching and/or DLC off SD. Both games will share the same cover legendary Pokémon, up to and including the forme itself. The game will feature another, previously unseen new town or city, to introduce the player to the new Unova, two years after the events of B2W2. Said town or city will involve the use of Generation VI-style view ports to give the player a sense of depth, and give the town or city a unique feeling, compared to anything before B3W3.
B3W3 will drop Team Plasma and introduce a single villain character, very unlike other games, though there are other "teams" to distract you, as well as the villain himself, from time to time. The game will rely a lot more of set pieces in the story, cut scenes abound. The new villain has something personal against N, and we do not know the true identity of said villain until after the game ends, with the villain disappearing, permanently, due to unknown circumstances.
The game will end with the story, and not end with the League, which should serve as a throwback to BW itself. The game will probably be a love letter to old fans, and fans of the Unova games in general, with treats for everyone to see, hear, play, laugh at, and be generally memorable, just like what a good movie should do. There will be at least two climaxes in the story of the game, with one reserved for the final confrontation, right before the credits roll, involving people that everyone liked the first and second time around, all together, to put an end to all of this.
Generation VI-style battling and other mechanics will be in use in the games, but tweaked to ensure better balance in general, and it will work especially well with pre-existing Unova Pokémon. Battle graphics will get further refinement in B3W3, as it was the case with B2W2 compared to BW, with key being on the backgrounds. Generation V-specific Pokémon will get a lot of attention in B3W3, not unlike the original games, and a bit unlike B2W2. Extra animations will be present in B3W3, compared to XY, and to a Pokémon player, this is probably what's closest to the game looking like animation that came into life the most, before the next game, of course. All the graphical quirks in XY will be either embraced or smoothed on, increasing visual coherency.
Lots of old places will look like the "old new thing" - it's instantly recognizable, but it's also hugely revamped for the most part, to take advantage of the new view port in the Generation VI games. Many old things in B2W2 will remain accessible, though that does not rule out unpopular areas getting blocked off for story reasons.
"You know... It seems like your cape's leaking. That's beside the point, though. Why you? I probably can deal with you later."
"... ... ... ... ... ..." (Ghetsis stays silent.)