Pokémon's biggest missed opportunity is still its dropped GS ball plot
The Pokémon anime has occasionally stumbled when it comes to handling Mythic Pokémon, with the Abandoned GS Ball plot being the biggest example.

Throughout its long history, Pokémon anime has done many plots and storylines, but not every one of them has a satisfactory ending. The GS Ball episode is one of the most notable ones that suddenly disappeared, providing the impetus for Ash to continue on to the Orange Islands and then to Johto.
In "Pallet Party Panic", right after Ash's loss in the Indigo League, Professor Oak asks Ash, Misty, and Brock to travel to the Orange Islands to retrieve a "GS Ball", one of his colleagues they got the GS ball, but Ash then spent time in the archipelago, catching Pokémon and challenging the local gym, so the mystery of the GS ball had to wait. After the chapter of Pokémon's Orange Islands, Xiaozhi gave the GS ball to Professor Oki, but Xiaomu didn't make much progress in research, but asked Xiaozhi to bring it to a man named Kurt who lived in Chengdu Poke Ball expert. It took a while, but Ash eventually found Kurt and delivered him the GS ball...after telling Oak the ball had been delivered, It was never mentioned again.
GS Ball is clearly meant to have some kind of greater significance, but whatever it means the significance never comes up and the audience never gets an answer to its mystery. However, fans can infer a few things from the Pokémon Crystal. There, the GS Ball is an item that can only be obtained through a special live event; when brought to Kurt, he inspects it for a while before returning it. If the player takes the ball to the Hollywood Temple, placing the ball in the temple will cause the rabbi to appear, allowing the player to capture the mythical Pokémon. Since GS Ball has a strong connection to Celebi in the game, it's reasonable to assume that's the case in the anime too...but why the sudden break?
The Pokémon Movies Were Prioritized Over the Anime

In a 2008 interview with Poké Beach's "Water Pokémon Master" and former director of the Pokémon anime Masamitsu Hidaka, Hidaka was asked about the GS Ball. In the interview, Hidaka said that GS Ball does in fact contain Celebi, or at least will take into account the Celebi-based arc in the series. then The first Pokémon movie had only been released for a few months before the launch of GS Ball, and it's unclear whether Pokémon movies will become the norm. However, when GS Ball was written out of the show, the release of the third film was imminent and the fourth film was on the way. Hidaka said that it was decided to have Celebi appear in the fourth movie, which was eventually titled "Pokémon 4 Ever: Celebi, Voice of the Forest", so in order to avoid complications, GS Ball made the appearance of Celebi in It's been written before in the series. Hidaka said they hoped fans would forget about the GS Ball...which didn't really happen because he still gets questions about it eight years later.
This decision eventually laid the groundwork for the Pokémon movies, which have since then focused on mythological tales, preventing most of them from appearing in the series, and forever changing the anime's list of creatures that can and cannot appear. Had the GS Ball arc played out differently, today's Pokémon anime could indeed be very different.
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