Details are under lock and keyblade for now.
Tetsuya Nomura has spoken about the future of Kingdom Hearts 3 and the whole series in a recent interview with Dengeki Online (via Videogamer), the series director explained that Kingdom Hearts 3 will be receiving free updates and premium DLC that could arrive by the end of this year.
Speaking about the free updates, Nomura said that “free DLC will be issued in the form of additional updates,” that would appear as and when they’re ready. Though the plans for paid DLC are different, with Nomura saying that the plan is “to put out several elements at once as a batch,” before suggesting that we could see this first batch by the end of the year.
As you’d expect, details on what the DLC would be about were pretty light, though Nomura did say that plot points concerning characters like Xion would be resolved in the DLC.
“As for the next Kingdom Hearts, there will be at least one KH title that will release before a ‘KH4’” #kh3ultimania— Kingdom Hearts Union (@khunion) February 28, 2019
Nomura also alluded to future plans for the Kingdom Hearts series, saying that the future of the series is “blank paper,” for him and his team now. Though a tweet from Kingdom Hearts Union quotes the Kingdom Hearts 3 Ultimania book that suggests we;ll see at least one more Kingdom Hearts title before Kingdom Hearts 4, which will probably arrive in 2086.