The producer of Pokémon Masters Yu Sasaki posted an open letter about the game on the Pokémon Master's official website. The letter addresses the upcoming changes to the game including: the Costume Event:

A Message from the Pokémon Masters EX Team (Vol. 27)

Greetings, Trainers. We’re the producers of Pokémon Masters EX, Yu Sasaki and Tetsuya Iguchi, here with a new message for you.

Today, we’ll focus on details about the next Villain Arc updates, upcoming story events, and more.

App Update for Late November

Hall Skip Feature Added to the Battle Villa! The Hall Skip feature has been added to the Battle Villa. In the next Battle Villa challenge and the ones that follow, once you have completed all Halls (excluding co-op), you’ll be able to skip to the Hall of your choice during the next Battle Villa challenge after that. When skipping, you’ll be able to choose how far to go—up to either the fourth, ninth, fourteenth, or nineteenth Hall.

You won’t be able to collect bonus rewards, Coins, or XP as completion rewards for Halls you skip, but you can collect their first-time rewards all at once.

Please note that if you don’t complete all Halls in the next Battle Villa challenge, you won’t be able to use the Hall Skip feature in the Battle Villa challenge that follows.

Try completing all the Halls each time the Battle Villa opens! For more details, please see the next Battle Villa notification.

Play Legendary Events Anytime!

Legendary Adventures have been added to the Main Story menu, allowing you to play previous Legendary Events whenever you want.

At the time of the update, only Giovanni & Mewtwo: Lurking Shadow is available, but more stories will be added in the future.

For those of you who have already completed the event and received all of the rewards, you can exchange the rewards you get for 5★ Power-Ups and other items, just like with legendary events that have returned up to now.

Champion Stadium Update

Starting with the next Champion Stadium challenge, the upper limit for Master Mode’s total point rewards will be raised to 10000 points, and Champion Spirit and Battle Points will be added as total point rewards.

To coincide with the update above, the maximum number of points that can be earned in a single battle has been changed from 1500 to 2000.

Enter the Champion Stadium, and try to go for 10000 points to get all of the total point rewards!

The New Eggstreme Pass Debuts

Eggstreme incubators are now available at Bellis’s Lab. To use an Eggstreme Incubator, you can obtain an Eggstreme Pass from the Purchase Bundles section of the Eggs menu on the Shop screen or from Bellis’s Lab.

Upon obtaining an Eggstreme Pass, you will have access to a limited-time Eggstreme Incubator for 14 days. During this period, Eggs placed in the Eggstreme Incubator require only half the incubation time.

They’re great for efficient Egg hatching, so we hope you make use of them for your future adventures. Please see the notification for more details regarding Eggstreme Pass updates.

Other Updates

When both the Game Text Language and Trainer Voice-Over Language settings are set to Japanese, the Sync Pair viewing feature will now display the names of the Trainers’ Japanese voice actors.

Grimsley (Kimono) & Bisharp Debut!

Grimsley (Kimono) & Bisharp will appear in the Sync Pair Scout that is scheduled to begin on November 25, 2021. Grimsley (Kimono) also appears in the story of the current Costume Event, so if you haven’t played it yet, please give it a look!

Plus, another new event will start in late November that also features a new Sync Pair, so we hope you look forward to that as well.

Grimsley (Kimono) & Bisharp

Grimsley (Kimono) & Bisharp are a Steel-type Tech Sync Pair. Their Stay Sharp! Move lowers their Defense and Sp. Def by six stat ranks, but it also raises their Attack by six stat ranks, raises their critical-hit rate by three stat ranks, and grants them the Supereffective ↑ Next and Enduring effects.

In addition, when they use Metal Burst, which is a powerful physical Steel-type Move, they take up a counterattacking posture. If hit by an attack move while in this counterattacking posture, they will leave the posture and attack the opponent that hit them. The more damage they take, the more powerful Metal Burst will be, so try using Stay Sharp! before Metal Burst!

Their Grimsley’s Gambit Passive Skill works very well with Metal Burst because it makes opponents target them for a short time when using Metal Burst. In addition, after the sync pair successfully attacks with Metal Burst, Grimsley’s Gambit will return their lowered stats to normal and raise their evasiveness by six stat ranks.

The risk-taking Grimsley is no stranger to putting it all on the line, so aim for massive damage with Metal Burst!

Villain Arc is Featured in the Paulo Interlude

A Paulo Interlude, which is an additional story snippet focused on Paulo, is scheduled to be released on November 25, 2021.

As of now, in order to play this new story, you must complete all parts of the Kanto chapter on Normal difficulty, so if you haven’t played the Kanto chapter yet, head to the Main Story menu and give it a try.

Further Paulo interludes will be added between Main Story chapters, so we hope you look forward to the next addition.

New Seasonal Sync Pair Debut

One male Trainer and one female Trainer will appear as sync pairs in seasonal outfits starting December 15, 2021. We hope you look forward to seeing which Pokémon these Trainers form Sync Pairs with.

To go with these Sync Pairs, there will be another seasonal event this year. We are also planning to bring back a seasonal event from last year, so we hope you enjoy it.

Future Updates

Finally, we’d like to share some information about future updates.

New Super-Challenging Content Is Planned!

As we mentioned in our previous message, a new high-difficulty event called the Legendary Gauntlet is scheduled to begin in mid-January next year.

The Legendary Gauntlet is a special event in which you take on multiple challenging Legendary Arena battles to see how many times you can win in a row.

Sync Pairs that you bring to one battle cannot participate in any other battles regardless of their battle’s outcome, so the key to success here will be to train a diverse array of Sync Pairs and use them to their full potential.

Also, completing this event will get you a special reward—a new kind of Lucky Cookie. Using these new Lucky Cookies will let you learn various Lucky Skills, including new Lucky Skills that will be introduced to coincide with this event.

The Lucky Skills a sync pair can learn will vary depending on their number of ★s, but of course, the more ★s they have, the easier it will be for them to learn more powerful Lucky Skills. We hope you’ll give this new event a try!

The Next Main Villain Arc Update

The next chapter for the Villain Arc is scheduled to launch in mid-January of next year.

We’re planning to release a Villain Event Prelude in late December to get you ready for this update, so we hope you look forward to it.

What to Expect in the Next Message

Our next message is scheduled for late December. In it, we plan to share information about new Sync Pairs, the Legendary Gauntlet, and an update related to the game’s background music.

We’re always working hard to deliver updates and improvements that make the world of Pokémon Masters EX as fun as possible.

We prioritize Trainers’ feedback throughout the development process, so don’t hesitate to let us know what you think.

Until next time, thank you for playing Pokémon Masters EX.

Pokémon Masters EX Producer: Yu Sasaki
Operations Producer: Tetsuya Iguchi

Note: The content mentioned in this message is currently under development. Titles, plans, update content, and update times are all subject to change.

Source: https://pokemonmasters-game.com/en-US/announcements/Other_3100_4W_1