Although they're only a few months from release, there still isn't a ton known about Pokemon Brilliant Diamond and Shining Pearl. One new detail might be coming soon, however, as according to Nintendo France, the games won't be featuring any new Pokemon.
First reported by Nintendo Smash, Nintendo France clarified in a comment that Pokemon Brilliant Diamond and Shining Pearl will be featuring Pokemon up to the fourth generation. While the company doesn't specifically say that it won't have any Pokemon from Gen 5 onwards, the specificity of saying that Pokemon "up to Gen 4" seems to imply that only the first four generations will be included.Pokemon Brilliant Diamond and Shining Pearl
It's important to note that, while this information seems to make some logical sense, it can't be confirmed entirely just yet. For starters, Nintendo Smash doesn't cite a source for its information except for an image of a comment by Nintendo France. What's more, the Nintendo Smash article claims that the comment was made by the Nintendo France Facebook page, but the image clearly shows that it was taken from a screenshot inside the Instagram app. This inconsistency could be because of the fact that Facebook owns Instagram and that the two are sometimes synonymous, but there doesn't seem to be a comment matching Nintendo Smash's photo on the Nintendo France Instagram page, so it seems like it was potentially deleted.