Pokemon Brilliant Diamond and Shining Pearl may not include Pokemon past the franchise's fourth generation.
A comment from Nintendo France on Facebook (via NintendoSmash, translated via Google) says that the games will allow players to "rediscover all Pokemon up to 4G," as well as the Sinnoh region's Underground and Team Galaxy.
That comment suggests that Pokemon from Black/White, X/Y, Sun/Moon, and Sword/Shield might not feature in the game at all, which would be a departure from previous remakes: FireRed/LeafGreen, HeartGold/SoulSilver, and OmegaRuby/AlphaSapphire all offer some means to acquire Pokemon from future generations. The only meaningful exceptions are Let's Go Pikachu/Let's Go Eevee, which limit players to the original 151 Pokemon, plus Gen VII mythical Pokemon Meltan and Melmetal.
Previous remakes do require players to reach a certain point in the game before they get access to expanded, 'National' Pokedexes, so it's possible that'll be the case again here. In the original Diamond and Pearl games, players gained access to the larger dex after encountering 150 different species of wild Pokemon. The way this comment is worded, however, does appear to suggest a harder limit.
Pokemon Sword and Shield were the first games to significantly limit the National Pokedex, originally cutting the full collection of Pokemon down from almost 900 to just 400. An expansion pushed that figure up to 600, but many fans were disappointed in the decision, dubbed 'Dexit' by the player community. The impact might be less keenly felt in a remake rather than a mainline game, but we'll only find out for sure when Brilliant Diamond and Shining Pearl release on November 19.
The Gen 4 remakes should keep you busy just long enough for Pokemon Legends: Arceus to arrive.