The way I see it, there are two big possibilities for these games:
1) They play the role as "Third installment", split into two (like Yellow, Crystal, Emerald, Platinum).
2) They play the role as "R/S remake", which would imply Hoenn.
Note that the first option would allow GameFreak to crank out another pair of games in the future, while the second would justify the split more. I mean, in case there is any reason for splitting the third installment other than money. They better add loads of new content if they want to split the games in two (but they won't be too different either).
Either way, do NOT expect many new items. A key item or two, yes, since those don't affect backwards (or sideways) compability with B/W, but nothing new and major. The same goes for triple typing. Never mind that the current coding doesn't allow it, Pokémon with three types would be too overpowered compared to the rest. Triple STAB would make coverage a whole different matter, allowing some Pokémon to hit hard everywhere. And again, backwards compability. The same goes for new abilities and moves. Freeze Shock and Ice Burn, that's all there is. Though, accuracy or perhaps even PP changes for some moves is likely. New effects to some abilities too, perhaps.
If, and I say if, GameFreak decide to give the new Kyurem Formes some Dragon-like features, my money is on the Adamant and Lustrous Orbs. Or something like those.
Also don't expect them to go away from the tried-and-true formula of "Kid goes on an adventure to beat the Pokémon League, stumbles across evil organization on his way". Any differentiations from this is reserved for the spin-off games.
I'm being a killjoy here, but don't expect to have any choices that would majorly influx the story line. Difficulty, choosing which region to start in... this would require GameFreak to code the same game twice.
Of course there will be move tutors and such. Even little kids would turn the games down if there was nothing new to do in the sequels. We will get a bunch of new move distribution, there WILL be whining (this last bit is a derivative of Murphy's Law), and some of us might even risk having fun.
Now, I guess this post will be the last on its page, so nobody is going to read it...