It was announced during a recent Dragon Quest event in Japan (Summer Festival).
Looks like the game will be revealed in the near future.
Source
At the end of the tournament, Taichi Inuzuka, producer of the Dragon Quest Monsters 2 remake and numerous other spin-off games of the series, announced that a new game is in production.
While there weren’t any details about the game or a title, Inuzuka said, “We’re working on a new Dragon Quest Monsters game, and its competitiveness will be as fun as always.”
Looks like the game will be revealed in the near future.
Source