Next year is the 20th anniversary of Kingdom Hearts.
Tetsuya Nomura already said last year in several interviews with Famitsu and Dengeki that the KH Team is preparing various things and that they hope to reveal it by 2022, when it is the anniversary of the series.
In addition, it seems that the title "An Oath to Return: KINGDOM HEARTS" would be the name of the next installment, since it appears in the secret ending of KH3: -ReMind- and in KH: Melody of Memory (something similar had happened with KH: Birth by Sleep in the secret ending of KH2: Final Mix).
Just a reminder. XD