The trailer introduces two new characters, Tsurugi and Asahi, two members of a research group called Project Mew who are, predictably enough given their name, searching for Mew. Tsurugi (ツルギ) has an Urshifu and will be voiced by Hiroshi Kamiya (神谷浩史), who recently voiced Corey in Mewtwo's Counterattack Evolution. Asahi (アサヒ), whose partner Pokémon wasn't shown in the trailer, will be voiced by Ryoko Shiraishi (白石涼子), who previously voiced May's Beautifly and its pre-evolutions.
Several fan favourite characters are also making a return to the series. Professor Oak appears to have some connection to Project Mew, introducing Ash and Goh to the project when they visit his Laboratory in Kanto and letting them know the team is currently recruiting members. While there, Ash meets up with many of his old Pokémon. Though it's not clear if this will simply be a casual visit or if we can expect many of these Pokémon to travel together with Ash again, the trailer does at least show Ash, Goh, and Gary Oak together encountering a Moltres, with Ash's Infernape engaging the Legendary Pokémon in battle.
Speaking of Gary, the trailer implies but doesn't state outright that he has joined Project Mew as well, showing his introduction amongst scenes of Ash and Goh heading to a snowy mountain, presumably in the Crown Tundra, where Tsurugi's Urshifu saves them from an attack from Regice.
Ash's companion through Unova, Iris, also makes a long-awaited return to the anime. Though it's not made explicit in the trailer, the Japanese PR release accompanying this latest trailer confirms that Iris has actually become the Champion of Unova during her time away from Ash, and the trailer also shows her wearing the same outfit she wore as Champion in Pokémon Black 2 and Pokémon White 2. Iris has apparently joined the Pokémon World Coronation Series, though despite being the Unova Champion has seemingly only reached Great Class so far. She and Ash face off Dragon Vs. Dragon in an official match, with both Trainers aiming for promotion to Ultra Class, Iris using her Dragonite while Ash leads with Dracovish.
The trailer ends with Goh reacting in surprise to something seen on the snowy mountain, before Mew's cry echoes over the series logo.
Pocket Monsters airs in Japan every Friday at 6.55 p.m. JST on TV Tokyo.
The folks over at the Solanime have helpfully subtitled the trailer in English, which you can view below.