Well I think a lot of Sinnoh's Plain color palette is due to fact it was based off the Hokkaido region, and that region known for farmland, so its going to be very plain in contrast to Kyushu's tropics, which is what Hoenn was based off of.As for ORAS, well, it's a personal opinion of mine that Hoenn looks better than Sinnoh, partially due to the latter having to make too many concessions to achieve that semi-3D style. That means that terrain shapes had to be very simplistic, buildings were largely cuboids, there were fewer types of trees, and simpler terrain textures. Hoenn had great detail in its overworld art style because it was all sprite-based. Sinnoh used 3D elements, and those had to be made quite simple for the DS to be able to process them. Just look at the functionally identical locations of Ever Grande City and Sinnoh's Pokémon League, for instance. Or the top of Mt. Pyre vs. the top of Mt. Coronet. There is a level of detail, scale, and complexity in the terrain of Hoenn you simply don't find in the terrain of Sinnoh.
Anyway, there's also another reason why remakes are done in the first place, with the exception of Let's Go, which was not a traditional Pokemon game. Remakes are often made with the intent of allowing Pokemon to be obtained the sane generation without the aid of previous hardware. FRLG were made with the intention of allowing players to obtain Kanto and Johto Pokemon missing from RS while Emerald allowed you obtain the missing Johto Pokemon not in FRLG should you not have Colosseium or XD.
HGSS, in addition of allowing the capture of Johto and Kanto Pokemon, also allowed you to obtain Hoenn starters and Legendaries because the developers specifically wanted to remove reliance on GBA games for completing the National Dex.
Between XY and ORAS, every Pokemon can obtained in Gen 6 without trading.
With Dexit and the removal of the National Dex, SwSh will never have to rely on another pair of games to complete their Pokedex. And this might be an unpopular opinion, but that's why I think that was the primary reason why remakes are made in the first place. DP also had at the time, the largest amount of available Pokemon in the entire series up to that point, with 445 available between both games out of 493. The majority of missing Pokemon were either starters, Legendaries, and mythical Pokemon. Its pretty much opposite of how SwSh excluded half of all existing Pokemon until the DLC hit the world. A remake of DP would require all these Pokemon to be programmed in the game in order to be faithful, which goes against the current design philosophy. I'll only consider getting them if they include all the missing Pokemon to SwSh, and those Pokemon can be traded to SwSh.
And here's definitely an unpopular opinion. Sinnoh remakes should have come out before Hoenn remakes, or at the very least at, during Gen 7. Why? Sinnoh was built around a touch screen. Minigames like Poffins, Poketch, underground, contests, Wifi plaza, all required use of the touch screen. Trying to translate these into Switch would be very difficult due to the lack of a touch screen- can imagine the underground mini game on the switch? The most logical replacement would probably be motion control, but can you imagine doing poffin making and the underground mini game with motion controls? That sounds so soreful for your wrists. And there's the issues of translating that to handheld mode. In conclusion, DP have a lot features that relied on touchscreen and don't simply translate well to switch. Trying to remove these features would strip Sinnoh of its identity and not be a faithful remake.
On the other hand, Hoenn not built around of a touchscreen, so a lot of its UI and mini games wouldn't be so hard to translate onto Switch. Even the battle UI resembles the GBA days. In my opinion, the best time for a DP remake would have been Gen 7, since it was the last games to feature touchscreen and every Pokemon in existence ( Should have given us DP remakes instead of USM and Let's Go. )
I also Yung Dramps with that its unlikely for DP remakes to celebrate the 20th Anniversary. They purposely skipped over XY followups to profit off the hypes of a new game rather than give XY its third version, nothing has happened since then to make me think that anything would change now.