With the release of Pokemon Brilliant Diamond and Shining Pearl, as well as Pokemon Legends: Arceus, there has been a lot of focus back on the Sinnoh region in Pokemon recently. The return to one of the more popular regions in Pokemon has been shown in the games, but the anime will also be joining in the love for Sinnoh. A new opening song and animation will be premiering within a few episodes, and an old rival for Ash will be making his reappearance soon.
Pokemon Journeys: The Series, the current arc in the Pokemon anime, has been heaping on some fanservice by bringing old characters back for cameo appearances. In particular, Dawn has made a number of appearances in recent episodes of Pokemon Journeys, and she was one of Ash's old companions on his journey through Sinnoh. She is one of the anime's more popular traveling companions with Ash, with fans enjoying her growth as a character and determination through her various Contest trials and Pokemon battles.
In the new opening for Pokemon Journeys, a new version of the regular opening song, "1・2・3," featured a number of new animations, including a callback to Ash's old signature shot of him pulling his cap backward and zooming into his right eye before he throws his Poke Ball. Near the end of the opening, Ash's old rival from Sinnoh, Paul, makes a brief appearance. The Twitter account devoted solely for the Pokemon anime, also tweeted a screenshot of Paul. It also mentions how it's Paul's, or Shinji as he's known in Japanese, first appearance since the end of the original Diamond & Pearl arc of the series.
The anime also looks like it will be continuing to make throwback calls to the series' past. In the scene right after Paul appears, Ash's Greninja will also look like it will be returning as well. Ash's Greninja is one of his best Pokemon, with some fans believing that Ash's loss in the Kalos Pokemon League finals was simply due to the scriptwriters deliberately messing up Ash's chances at a victory. Ash's Greninja lost to Alain's Charizard, a loss that any Pokemon player will tell you is extremely unlikely to ever happen.
As the anime itself approaches its 25th anniversary, it seems to make sense to start getting people on board the nostalgia train. The Pokemon anime is well over 1000 episodes now, making it among one of the longest-running anime series in history, even if its different arcs/seasons technically don't make it one continuous anime. For many people, the first couple of seasons will remain a big part of their childhoods, as the Pokemon cultural phenomena continues to evolve and attract new fans.
With all of this focus on Sinnoh, it leaves some fans wondering if the Pokemon series will revisit other regions from the past perhaps in other "Pokemon Legends-style" games. While the focus on the near future will be on Pokemon Scarlet and Violet, the door does remain open for the franchise to show some love for regions from the past.