Kingdom Hearts 3 isn't over just because you beat the game. Director Tetsuya Nomura has confirmed that both free and paid DLC is in development and scheduled to come out in 2019.
In an interview with Japanese game publication Dengeki Online, Nomura revealed that there will be both free and paid DLC that should be out before the end of 2019. Nomura says in the interview, "I think that free DLC will be issued in the form of additional updates," while paid DLC will come out in bigger packages (via Google Translate).
For example, in regards to specific narrative threads the DLC can pick up on Denagki asked Nomura about some spoilerific plot points in Kingdom Hearts 3. To which Nomura teased that he, "will explain it with DLC." Considering how sprawling Kingdom Hearts 3 is, there's a lot of different directions the paid DLCs could go and explore.
There's no release date or schedule for any of the DLC Nomura mentions other than a 2019 timeframe. Let's hope Kingdom Hearts 3's DLC doesn't go the way of Square Enix's Final Fantasy 15, which was forced to cancel a slate of DLC storylines recently. It'll be cool to see where Nomura and co. take the larger DLC segments. Perhaps it will even introduce new worlds or characters, possibly as tie-ins for new Disney properties.
You can read our Kingdom Hearts 3 review for our take on the latest Disney/Square Enix crossover or check out our Kingdom Hearts 3 guide for a walkthrough. If you finished your adventure, check out our Kingdom Hearts 3 Spoiler FAQ for what storylines the DLC could potentially expand on.