The Masters Eight lineup contains a mish-mash of nostalgic characters, however, their inclusion does nothing to add to the Pokemon Journey's storyline, whereas other Trainers would have been a much better fit.
The Masters Eight line-up has a strange assortment of trainers and, although all skilled, they do not represent the best of the best in some fans' eyes. Leon and Cynthia have proven their worth as potent Pokemon trainers with an extensive fanbase and their inclusion in the top two spots of the Masters Eight, while Alain represents the anime-only aspect to the line-up and brings up the rear. Ash's inclusion is a no-brainer, and his battle against Steven Stone has been anticipated for quite some time, however, the rest of the top participants have inspired some head scratching.
The Expendables
Iris (#7 Rank)
Out of all the female Trainers throughout Ash's travels, Iris has gotten the least attention from the Pokemon community, and if she is ever mentioned, it is generally not in a positive light. Her character design was intended to be edgy and fun, as a disorderly princess-type raised in the Village of Dragons among the wild Pokemon (sound familiar? Tommy, Koko...), who added a bit of diversity to the relatively whitewashed cast. Unfortunately, Iris is more successful at irritating the viewers as much as she did Ash, and her forgetful Axew leaves much to be desired. Perhaps her reintroduction into the Masters Eight was an attempt at redemption, but Iris' infamously vexing "You are such a kid" phrase is unintentionally ingrained into fans' minds.
Diantha (#5 Rank)
Another regularly overlooked and often forgotten Trainer is Diantha, who arguably plays a larger role in the Pokemon video games than making any significant contributions to the anime. Although she is present during the first-ever Ash-Greninja transformation, everybody is well aware that Diantha is a movie star first and foremost and that Pokemon Training is not her top priority in life. As more of a showpiece, Diantha elevates the aesthetic factor along with an interesting team of strong Pokemon but adds little to the overall Pokemon Journeys storyline.
Lance (#4 Rank)
Another Master of Dragons joins the top eight contenders, saturating the variety of Pokemon even further with more wyvern-inspired creatures. Lance has also already faced Leon and lost, and while everyone deserves a second chance, does that warrant receiving such a prestigious position in the World Coronation Series? He has not left much of an impression on fans since the Team Aqua and Team Magma arc (in Ruby & Sapphire), and his previous Kanto/Johto Champion title was conspicuously absent during his Masters Eight introduction speech, suggesting that Lance is not as skilled as he once was.
The Replacements
Serena: (Possible #7 Rank)
While it is unlikely that Serena would be proficient enough to proceed past the semi-finals, creating additional opportunities for Ash and this lovely lady to interact would be satisfying enough in its own right. While fans recently enjoyed spotting Serena in Pokemon Journeys, her contribution to the storyline thus far has had a greater impact on Chloe rather than Ash, leaving many fans begging for a proper reunion.
This pair of prospective lovebirds could cheer each other on from the sidelines of the Masters Eight tournament, even eventually squaring off in a battle, and potentially posing a moral dilemma in the process. Working through the emotions of fighting against such a dear friend would be the perfect, seamless opportunity for Ash and Serena to finally declare their feelings for one another. Sure, it might not be what the writers want, but as one of the most prolonged romance stories in anime history, it's time to give the fans what they need.
Gary (Plausable #5 Rank)
Although Gary made an appearance in episode 68 of Pokemon Journeys, he is also pawned off onto Ash's companion, but this time with Goh on the receiving end (as if fans had forgotten that he is Ash's first and most notorious rival). It would seem that Gary is now more interested in Project Mew than the World Coronation Series, adding to Goh's rather insignificant storyline instead (way more fans are discussing the Masters Eight than Professor Oak's passion project.)
Yes, Gary did officially step away from Training in "Johto Photo Finish," but Pokemon Journeys features so much fan service that it seems a wasted opportunity not to rekindle this rivalry in an epic, absolute final stand-off. Gary's skills clearly haven't gotten rusty because he practically stole the show in the battle against the legendary Moltres in "The Road to Mew!!" It appears the relentless Gary has been training against Ash's fire-type Pokemon (left in Professor Oak's care) for some time now, with visible improvements.
Gladion (Potential #4 Rank)
Even though Gladion is a relatively new character in the Pokemon franchise, he has left quite an impression as a relatively well-rounded individual with a rich backstory and mysterious aura that has viewers begging for more. The main antagonist of the Sun & Moon series may have a serious demeanor but is still considered to be one of the best game-to-anime crossover characters. And it seems that representing the video game counterparts was a major priority for the Pokemon Journeys writers, leaving little reason why Gladion should not be included.
Gladion has a knack for pushing Ash past his limits and encouraging him to improve his skills at every opportunity. His life lessons could be a crucial component to Ash's potential success, urging him towards the Monarch title, in that backhandedly-friendly way Gladion does so well. The added interpersonal competition would also create more drama in the Masters Eight tournament's storyline (which has already received some critique from fans.)
Regardless of any Masters Eight rearrangements, the entire World Coronation Series seems to be geared towards amping fans up for the highly anticipated battle between Ash and Leon. Cynthia has a good leg to stand on regarding her claim to the Monarch title, but everyone else gives the impression of filling in the gaps for the sake of fanservice while attempting to strengthen the link between the Pokemon video games and anime.