Someone already mentioned Serena’s Pancham, but I’ll say it again. Back when it was first spoiled/revealed (can’t remember which, has it really been 6 years?) I was ecstatic; not just because Serena was finally catching something but also because I really loved Pancham and was so glad a “tougher”- it’s cute and marketable, yes, but also spunky and more a Pokémon I figured they’d give to Ash- catch was going to her. I hoped they’d have it evolve to Pangoro so it could be her Mamoswine/Blaziken/Dragonite equivalent, even though realistically it’d stay small plushie bait. Even if a bigger Pokémon like Pangoro didn’t fit the typical Showcase image, with its personality and its love of performing I thought they could’ve done a really cool arc where he evolved and had to overcome that stereotyping and judgement with Serena’s support. Personally I would’ve loved to have seen a huge Pangoro just haul Serena across the stage, but alas.
Evolution aside, I also felt like Pancham just got the shaft a lot. Braixen and Sylveon get episodes dedicated to their character development, while Pancham’s whole character is just relegated to the obnoxious Chespin rivalry (which had no reason to exist). Battles were also done by the former two mostly. And when it came to relations between Serena and her Pokémon, she seemed to have way more genuine relationships with everyone but Pancham. It honestly felt like he had no reason to exist, which was really disappointing.