https://wccftech.com/pokemon-brilliant-diamond-and-shining-pearls-new-features-have-been-revealed/

So, we've got a few more updates to Pokemon Briliant Diamond and Shining Pearl in the latest Pokemon Presents... Presentation. Brilliant Diamond and Shining Pearl will have several expansions alongside returning features from games like Pokemon Sword and Shield. Some aspects of the game have been changed, too. So, let's talk about everything we know from the presentation.

First and foremost, we'll talk about the biggest update. The original Underground area has been expanded to become a Grand Underground. This area will feature Pokemon Hideaways that allow for the capture of Pokemon that weren't even present in the original games. Not only that, the Pokemon you encounter in the Grand Underground is based on the statuses in the player character's base, which you'll be able to upgrade as you explore.