Headed into the recent Pokemon Direct, fan demand for remakes of Pokemon Diamond and Pearl hit an all-time high, but to no avail. The show basically outlined how Pokemon content would be released over 2020, with a remake of the original Pokemon Mystery Dungeon coming to Switch and Pokemon Sword and Shield getting details for its post-launch support plans. This is the first time in franchise history that, opposed to a special third edition like Emerald or Platinum or a sequel like Ultra Sun and Ultra Moon, DLC will appear in a mainline Pokemon game. But that doesn't bode well for the future of Sinnoh.
As it stands, Pokemon Diamond and Pearl are the next entries in line to get remade, as these remakes crop up every 4-6 years or so and prior generations have already been explored. Fire Red and Leaf Green released in 2004, Heart Gold and Soul Silver came out in 2010, and Omega Ruby and Alpha Sapphire made their way to the 3DS in 2014. With that in mind, this pattern of release suggests the fan-desired remakes would hit in 2020, yet a few developments old and new completely throw this out the window.
First, Pokemon Let's Go Pikachu and Let's Go Eevee have to be added to the equation, namely because they are remakes of Pokemon Yellow, Pokemon Red and Blue's third edition released in 2018. If this is the start of a new trend, that means remakes of Pokemon Crystal and Pokemon Emerald are not out of the question, so instead of remaking Diamond and Pearl, it could be that prior generations' special editions will be getting the extra treatment. Now, one installment isn't necessarily enough to imply this to truly be the case, but as remakes falling into the 4-6 year aforementioned window, that could easily push Diamond and Pearl into 2022.
Of course, many do not see Let's Go Pikachu and Let's Go Eevee in the same vein as traditional remakes, but even factored out, the major change in tradition with Pokemon Sword and Shield's post-launch content paints a grim picture for Diamond and Pearl remakes. Game Freak is going to be busy over 2020 providing Sword and Shield players with new content in The Isle of Armor and The Crown Tundra. The Expansion Pass then delays Diamond and Pearl out of 2020, as there may not be enough resources to fuel all this development.
Looking over the past decade of Pokemon may shed further light on the situation. Black and White released in 2010, with its sequels coming out in 2021. X and Y launched in 2013 (to never have a special revisit) before Omega Ruby and Alpha Sapphire launched in 2014. 2016 kicked off with Sun and Moon, 2017 followed up with Ultra Sun and Moon, and 2018 hit with the Let's Go remakes. 2019 started the latest generation of Pokemon Sword and Shield which continues on into 2020.
This pattern shows remakes arriving after special editions (counting Let's Go), so with Sword and Shield set to dominate 2020 with DLC, then 2021 is logically the earliest it could arrive. And that's not considering the remake release pattern, just the core pattern. As such, expecting Pokemon Diamond and Pearl remakes to drop before 2021 or 2022 doesn't seem to be realistic.
Of course, with Sword and Shield's expansion pass tossing everything out the windows in terms of special editions, it could be that Game Freak surprises everyone by announcing it later this year. Having a Mystery Dungeon remake, a full DLC run for Sword and Shield, and remakes of Diamond and Pearl all in one year seems far less likely, though. It would be a fantastic start to the new decade for Pokemon if so, for sure, but that seems like a pipe dream, as is expecting these remakes any time soon.
Remakes of Pokemon Diamond and Pearl are not known to be in development.