This site, in general, has become more active. Amazing what a new game reveal, however small, can do for this bunch. ;)
Well, the confirmed compatibility with RBY could be a possible hint towards a simpler, more streamlined stat system. The number of new Pokémon will probably also remain low as in XY.
As a 20th Anniversary "throwback" to RBY, we could also see a return to some of the story/lore conventions of those games. For example, the mascots would be starters again, and the plot would be more mundane/realistic (but hopefully still better written and more fleshed-out than RBY's plot) and not center around a Legendary or saving the world. We could also have a Champion who's our rival again, or at least, anybody who doesn't fit the mold of "friendly older mentor figure who appears out of nowhere to help you fight the Evil Team" that GF has been recycling since GSC (hopefully the poor reception of Diantha was a wake-up call).
Either way, though, I hope SM are far more "complete" than XY were. If third versions and even sequels have become a thing of the past, then the main pair can't be half-assed and left unfinished. Imagine how Gens 3 and 4 would've turned out had Emerald and Platinum never been made, and Hoenn and Sinnoh were stuck with the broken, incomplete RS and DP, instead. That's exactly what has happened to Kalos.