Nintendo and The Pokemon Company revealed a bit of what to expect once you've beaten Pokemon Brilliant Diamond and Shining Pearl. After joining the Hall of Fame, you'll get access to a brand-new area, Ramanas Park, and all its legendary mysteries.
It's still a bit unclear what Ramanas Park is actually like, though the newest BDSP trailer shows it's full of caves, caves with altars missing certain tablets. Finding and replacing these tablets unleashes a set of legendary Pokemon in the wild, though in true Pokemon fashion, which ones you get depends on what version you play.
Nintendo Reveals Pokemon Brilliant Diamond and Shining Pearl Postgame Content
On the mythical side, you'll find Mew if you have save data from Pokemon Let's Go Pikachu or Eevee on your Switch system, and Jirachi if you have Sword or Shield data.
Anyone who purchases Brilliant Diamond or Shining Pearl before February 21, 2022, will receive a free Manaphy egg. Manaphy is unobtainable in the game otherwise.
Pokemon BDSP release November 18, though so far, we still don't know much about what to expect from the remakes as far as new content.