Pokemon Brilliant Diamond and Shining Pearl are going to be the remake versions of the originals, which were released in 2007 for Nintendo DS. Brilliant Diamond and Shining Pearl are not the usual remakes that all Pokemon games eventually get, and there are going to be substantial differences between these and any other Pokemon game ever made.
First of all, they are the first titles of the series to ever be produced by another company, and not by Game Freak. In fact, the studio is currently busy with another upcoming project that should release early in 2022. This is the first open-world Pokemon game to ever be made, and it is going to be called Pokemon Legends: Arceus. This game is going to be a sort of prequel title for the upcoming Pokemon Brilliant Diamond and Shining Pearl, and it is going to feature a feudal Japan style of setting, which is also a first in the history of the Pokemon franchise.
As for Pokemon Brilliant Diamond and Shining Pearl, the games are being developed by ICLA Inc., and they are going to be a close adaptation of the original games. According to Game Freak, in fact, the remakes are being developed with the team trying to maintain the original size and scope of things in Pokemon Diamond and Pearl.
While the games are seemingly set to be faithful adaptations of, it is still possible that they will feature newer mechanics from more recent games, such as Mega Evolutions or Z-Moves, or even bits of gameplay from Pokemon Platinum. The Platinum version was the third game that came out not long after Diamond/Pearl, with variations from the core games of generation four in the style of other games like Pokemon Yellow, Pokemon Crystal, and Pokemon Emerald.
While the differences between Pokemon Brilliant Diamond and Shining Pearl are purely speculation at this point, a good starting point would be to list the differences between the original Diamond and Pearl versions. In fact, with the new games being made to preserve the status quo of the original games, it is safe to assume that the differences between the titles could be the exact same.
Starting off, Pokemon Diamond and Pearl differed in terms of Pokemon available to be captured and trained, as all Pokemon games do.
In Pokemon Pearl, players could find:
In Pokemon Diamond, players could find:
Another difference between the games was that an image of Dialga would be seen in Celestic Town in Pokemon Pearl, while an image of Palkia was seen in the same spot in Pokemon Diamond. These images were used to let players have a Pokedex entry for both legendary monsters.
Pokemon Diamond and Pearl also differed in terms of encounter rates, and some critters being found in the wild exclusively in one version or the other. This doesn't mean that the games had each more version-exclusive Pokemon that were not listed above, just that some could only appear in either Pearl or Diamond. Examples of such Pokemon are Cascoon and Dustox can only be found as wild critters in Pearl, while Silcoon and Beautifly only appear in the Diamond version. Lastly, the games don't have many differences when compared in terms of the main story arc, apart from the story revolving around the characteristic legendary Pokemon for each version.
Considering that Pokemon Platinum is not likely to receive a remake as well, it is quite plausible that some of its features that initially differentiated it from Diamond and Pearl will be included in both remakes. For example, one of such features is that Rotom, originally available as its base Electric/Ghost form, received five new elemental forms in Pokemon Platinum. These forms all share the Electric type, but their secondary type changes from Ghost into either Fire, Ice, Water, Grass, or Flying.
Another difference that Pokemon Brilliant Diamond and Shining Pearl are likely to inherit is Giratina's secondary form, obtained when the legendary Pokemon is given the Griseous Orb. The same holds true for Shaymin, which was given its Sky form when it held the Gracidea.
The Battle Frontier is also likely to return, which was first introduced in Pokemon Platinum, but it bears a resemblance to the Battle Frontier found in generation three Pokemon games. There were five facilities here, each with its own final encounter, and Battle Points could be earned and then traded for several items, including TMs.
The story also changes a bit, and Cyrus, Team Galaxy's leader, unleashes a series of events that by the end allow players to visit a whole new area, previously not accessible in Diamond and Pearl. This is called the Distortion World, and no wild Pokemon are encountered here. This dimension does not follow the laws of physics, and it is Giratina's hiding place.'
Pokemon Brilliant Diamond and Shining Pearl will arrive in late 2021 on the Switch.