Pokemon OmegaRuby/AlphaSapphire were the only remakes that were really necessitated in my opinion, mainly because Hoenn was the last region in need of some type of three-dimensional (3D) conversion (in addition to the physical-special combat split being of great use within the Battle Frontier facilities, since some Pokemon playstyles were very different prior to said combat split - still hoping on more facilities to properly supplement the "Battle Maison"/Battle Tower of Hoenn)..
Additionally, most post-Pokemon Emerald core series Pokemon games seemed to have more useful movepools for Pokemon in addition to more reliable methods of using Technical Machine (TM) moves (compared to the paltry things like the one Earthquake TM you are permitted in Pokemon Ruby/Sapphire/Emerald, unless you somehow get lucky with a Pickup-Ability Pokemon), so it was great to see Hoenn's general experience enhanced in that regard.. Hoenn's vistas also seem to benefit the most from a 3D viewpoint (areas based on volcanoes, deserts, forests, jungles, industrial-looking cities, atop-water and underwater locales) although this is more of my own opinion..
I don't think that remakes of Pokemon Diamond and Pearl are warranted (particularly because Pokemon Platinum Version seemed to fix most of what was wrong in that regard); they even started out with quite a bit of 3D! None of the post-Pokemon Platinum innovations seem that integral for the Sinnoh experience (from my understanding, it still fared quite nicely without the Fairy-type and Mega Evolutions), and seem like they would be better suited for a new Pokemon region adventure instead. Aside from more Forme changes (which can really be done in any generation onwards) and 'nostalgia' reasons, the only other major change I can see is "prettying up the region", and I feel like the visual jump (at least in overworld terms) won't be as great as the 2D-to-3D makeovers for the regions of Kanto, Johto, and Hoenn, especially since Sinnoh's overworld already started out in 3D.
As for Kanto...yeesh. I've only really experienced Kanto via Pokemon FireRed Version, and already I'm tired of it. XD Pokemon HeartGold/SoulSilver accomplished the necessary 2D-to-3D Kanto makeover in my opinion, and Kanto can generally be traversed so that you still get a sufficient "Kanto" experience in a Johto-based game (even some of the Kanto scenery that Generation 2 couldn't handle was restored). Pokemon X and Y definitely didn't help matters for a "Kanto remake" by having a forestal layout mimicking that of Viridian Forest, in addition to sporting the Kanto starter Pokemon, Mewtwo, and the Winged Mirages/Legendary Bird Pokemon of Kanto. I mean, Kanto has appeared in at least 10 games (just counting English releases, and considering paired versions as distinct entities) within 4 consecutive generations...I know it's where the franchise started, but seriously, I know what Kanto looks like already. XD Anyway, I feel like a "back to one's roots" is appropriate for Pokemon franchise's upcoming 20th anniversary (but then that begs the question of if we go back to Kanto for the 25th, 50th, or any other "10*n"-th anniversaries >_>), although Kanto seems over-visited by now... I guess the "classic" Kanto might warrant another remake (since Pokemon FireRed/LeafGreen Versions are roughly as dated as the other Generation 3 games - Pokemon Ruby/Sapphire/Emerald Versions), although I'm really not enthused about it, and such a release would really have to make Kanto enticing (to people other than those who haven't played through the region yet, at least).
The core series Pokemon games would really benefit from Virtual Console releases, in my opinion - something like Pokemon Platinum would be great for the Wii U's Virtual Console, especially since DS games are now supported. In my opinion, the main issues that would still persist despite Virtual Consoles are lack of new Pokemon (which will be inevitable as long as we continue to get new Pokemon generations), multiplayer issues (sadly inevitable as well, as the online servers for Generations 4 and 5 have demonstrated with their shut-downs), and lack of "key" Pokemon Trainers (like Gym Leaders and the like). Although migrating Virtual-Console-obtained Pokemon would be nice (could be viable for all post-Generation 2 games, as long as the coding structure doesn't change again...?), it seems like the "regional experience/plot" would matter more; multiplayer battles and new Pokemon seem better relegated to the newer Pokemon releases. Honestly, an online browser-based "Pokemon" massively-multiplayer role-playing-game (MMORPG) could solve a lot of these problems. >_>
I just feel like at this point, we can't keep remaking Pokemon things forever - newer regions can still retain past-generation Pokemon by way of Safari Zone/"dimensional rifts and warps", and past-generation Pokemon can keep reappearing in battle facilities like the Pokemon World Tournament venues. I do feel like we need to maintain "backwards-compatibility" somehow (either by direct access, or by making past Pokemon games more widely and readily available on modern consoles through digital means, or by way of online MMORPG), so that newer Pokemon regions can be made and past regions can still receive their deserved attention.
Tl;Dr - Pokemon Diamond/Pearl/Platinum and other post-Generation 3 remakes aren't necessary; Pokemon Red/Blue/Green/Yellow/FireRed/LeafGreen remakes might be warranted because, hey, "20th anniversary hype"; Virtual Console releases would help mitigate the need for more "remakes"; "online Pokemon MMORPG" would really help out with things; experiences of past generations should still be readily available for players of the modern age.