Here are the first details, via Square Enix:
Kingdom Hearts Missing-Link allows players to embark on adventures from the realm of Scala ad Caelum into the real world. Players will be able to engage in exhilarating battles against the Heartless and discover a new, original story.
“We’re thrilled to celebrate the series’ 20th anniversary with the announcement of two new Kingdom Hearts titles,” said Kingdom Hearts series brand manager Ichiro Hazama in a press release—the other title being Kingdom Hearts IV. “We’d like to thank the fans for all of their support over the years, and we can’t wait for them to experience all that’s to come for Sora.”
Additionally, the company announced that the final chapter of the iOS and Android game Kingdom Hearts: Dark Road will launch in August 2022 as a free update to Kingdom Hearts Union χ Dark Road.
Watch the announcement trailer below (from 1:53 to 4:04). View the first screenshots at the gallery.