Hmm, I have mixed feelings about the news revealed about the film, to be honest.
On the one hand, they're actually focusing on Ho-Oh, which is a good thing since they haven't focused on Ash's connection with Ho-Oh at all since, what, early DP? And quite frankly, I can understand needing to retell Ash and Pikachu's first meeting since, let's face it, Ho-Oh played a small but significant role in the first episode anyways, so it's a good fit. And I must admit, I didn't expect them to actually distribute both Ash-Pikachu and Marshadow.
But on the other hand... I honestly think that it seemed completely unnecessary for Gary, Misty and Brock to be replaced with new characters to fill the same role. I mean, just because it's a retelling doesn't mean they should remove any and all characters who were involved in the original work besides Pikachu and Ash obviously, especially when they actually helped Ash become the trainer he is today, and helped Pikachu as well. To put it another way, the Dragon Ball franchise had a similar idea to the upcoming retelling of the first episode with one of their films: Path to Power (which in fact was created to celebrate the 10th anniversary of the Dragon Ball anime). It was a retelling of Goku's first journey. Like I Choose You!, it also featured significant changes to the plot (namely, instead of trying to find the Dragon Balls before Emperor Pilaf, he has to contend with the Red Ribbon Army). However, even with those changes, they kept his first meeting with Bulma, Yamcha, Puar, Oolong, as well as his meeting Master Roshi. In fact, they even further developed Bulma and Yamcha's characters in that movie than in the original arc it was based on. Heck, Path to Power was even released 10 years after the first episode of the Anime. They could have done the plot like the guys who did Path to Power did regarding their plot as well as especially the characters. And let's not forget, Misty and her fishing rod was the reason why Ash was even able to go to Viridian rather than end up a floating body at best. Heck, forget Misty or even Brock: Gary was Ash's longtime rival, and a relative to Professor Oak, and a large part of Ash's motives in Season 1 was to surpass Gary, so removing him was just stupid. Besides, they, Misty especially, haven't gotten any focus as of late, so it's best if they actually include them, especially since it is the 20th anniversary anyways.
Well, either way, let's see how the film does (and I suspect that it may be the last film in the series. since not only are they focusing on Ho-Oh, a Pokémon that formed the crux of Ash's character arc up to DP, but they even are debuting both Ash-Pikachu and Marshadow, the latter of which is the only Mythical Pokémon of Generation VII besides Magearna which already debuted in the Volcanion movie earlier. Hard for them to make another when they've already used up all of the Gen VII Mythical Pokémon available. I mean, they never had a movie without distributing a Pokémon, namely one of the stars of the movie and especially a Mythical Pokémon, since Pokémon Heroes fifteen years prior.).
I, for one, am overjoyed that we've got new characters
That means that Brock and Misty will actually be doing their jobs instead of ignoring them to tag along with some random stranger
I welcome our new Alternate Universe Ash and wish him the best of luck on his journey
...
Misty was already traveling by herself before she met Ash, all in an attempt to prove to her sisters that she actually had worth, so technically she wasn't actually active in her job beforehand anyway. And she certainly took the job far more seriously than her sisters did in either case and it was implied that the only reason she wasn't often participating was due to her age, being the youngest of the Cerulean Sisters. Heck, she even fought Ash when it seemed like her sisters were just going to give the badge away, and also protested how Ash got the Cascade Badge despite not actually winning against her, so she definitely took the responsibilities of the job pretty seriously and certainly didn't ignore it.
As far as Brock, he didn't even initially intend to go on a journey with Ash, or journey at all, because he felt obligated to take care of his siblings after his parents ditched them (or his dad, depending on which version you go by). He in fact specifically requested that Ash take care of his dream for him when giving him the Boulder Badge, and only went journeying at all, let alone with Ash, when his father, Flint, returned and encouraged Brock to pursue his dreams himself to make up for not being with his family. In fact, when Ash met Brock, he WAS focusing on his job as Gym Leader. He even had to fight Brock as the first Gym Leader.
Sorry for going into this tangent, but I get annoyed when people act like Brock and Misty ignored their jobs in the anime, even when that's far from the truth and a very shallow way of looking at it. Besides, like I said, it seems to be pointless to have Gary, Misty and Brock replaced with three characters who are going to fill the same role anyways. It would be simpler to actually have them be in the movie (and it wouldn't even necessarily hamper the film with their presence either. Don't forget, Path to Power did something similar, yet they not only retained the characters from the original anime besides Goku himself, they even gave them even MORE development than originally). Let's hope Misty at least has a role in the movie, since apparently they personally delivered the script to Mayumi Iizuka back in 2016 if the YMMV section for the I Choose You article for TVTropes is to be believed.