I feel like Gen 5 was the peak of the series aesthetically and design-wise but I also feel like its the hardest to return to. There are just so many defunct features (c-gear, dream world) and to really have a proper playthrough with memory link and challenge mode is an enormous hurdle that requires two additional games.
Diamond and Pearl are coming out and look closer to ports than actual remakes. With it being outsourced and gamefreak focusing on a new direction with the series with legends, I wouldn't be surprised if these games are never remade purely because of how difficult it would be to do so and them seemingly being past the nostalgia threshold.