A new generation of Pokemon comes out on a new generation of consoles. RBY - Game Boy, GSC - Game Boy Color, RSE - Game Boy Advance, DPPt - DS. The DSi doesn't count as a new generation, it has what, a camera? So we can almost definitely rule out the possibility of a 5th generation at this stage.
It also has more RAM, better online capabilities, and a faster processor. Its as much a new generation of handhelds as the gameboy color was, and keep in mind Gold and Silver were actually compatible with the old gameboys. Crystal was the only GBC-exclusive game in that generation, and personally I didn't get a GBC until Crystal came around. Also keep in mind, Nintendo has already stated their intention to have both DS games with DSi-exclusive features in addition to DSi-exclusive games down the road. For what its worth, there's already at least one game coming out with DSi-exclusive content, My Healthy Cooking Coach. Don't know what that exclusive content is off the top of my head, just know its going to be there. DSi also opens the possibility of downloadable missions through DSiWare and what not, if Nintendo uses the SD card compatibility for DLC.
The 5th generation games will either be DSi-exclusive, or have bonus features on the DSi and have the third game in the 5th gen be DSi-exclusive. Feel free to write this down somewhere as I stand by that prediction.
On a side note, I half expect 1st and 3rd gen legendaries to be showing up in these remakes, given the surprise appearance of the Regis and birds in Platinum. With those additions, its almost like they're trying to remove the need to have the GBA games. Not that the GTS doesn't handle that pretty well as it is. Including event pokemon, Platinum had 19 legendary pokemon to catch. 20 if you count Phione. To make the rest of the legendary pokemon available 4th gen, GS remakes would need to add 10 more legendary pokemon for a total of 16. Doesn't seem like it'd be too far out there for there to be another "Sevii Islands"-type area where a bunch of Hoenn pokemon are made available, alongside Mew's island and having Mewtwo appearing in Kanto somewhere. Though that would raise the question of why they had the birds in Platinum instead of maybe the Lati's...
...I wonder if the G/S remakes would have DSi-exclusive content? Surely they wouldn't be DSi-only though.