Pokemon games have been about as consistent as Law & Order for the last 15 years. You can pretty much set your watch by them. Not that that's a bad thing when the base game is so damn good. I'm playing through Y right now and enjoying it thoroughly.
These remakes will use the same graphical engine as X and Y to update the Ruby and Sapphire content. There will probably be some postgame content added for when you defeat the Elite Four (a new area like the Sevii islands from FireRed/LeafGreen, along with some kind of high-level battle area), as well as opportunities to catch other monsters from Gens I-V, likely including some of the past legendaries, in an effort to reduce reliance on transfers from the Gen V games.
As far as the incremental changes that Game Freak does see fit to put into the new games, it would be nice if they got rid of HMs, as has been speculated recently. Now that TMs in Gen VI are re-usable, there is no practical reason to saddle your team with low-power moves that can only be deleted by that one guy in every game.
Over the years I played the hell out of Blue, Yellow, and Silver. I skipped the GBA games, played Pearl and SoulSilver, and skipped Gen V before picking up Y. I'll probably get these "new" ones when they come out. It's easy to be cynical about the children's merchandising juggernaut Nintendo has carved out over the years with these prolific, incremental updates, but the games at their hearts are really solid RPGs that scratch that min-maxing itch in ways that no other series can. Furthermore, Game Freak hasn't gone down the release-a-broken-game-and-maybe-get-around-to-patching-it-when-enough-people-bitch road like Bethesda, nor the nickel-and-dime-a-season-pass-fee-with-content-already-programmed-into-the-physical-game approach of Capcom. For a series that has been about connectivity from the very beginning, Pokemon has ironically managed to avoid the pitfalls of DLC-Hell. Nintendo's Pokemon Box service has me a little nervous, but for the time being, Pokemon remains a top-class video game in an era where we aren't entirely sure what that term even means any more.