Speaking to Game Informer at GDC 2019, Yasue said:

Basically, the reason why there are no Final Fantasy characters in Kingdom Hearts 3 is because the team wanted to focus on Square Enix’s original characters instead. While that’s all fine and good, it does somewhat ignore the fact that huge portions of the game focus on Disney characters instead of the main conflict with Xehanort, and so some fans may still be unsatisfied with Yasue’s explanation.

While fans may still be disappointed about Kingdom Hearts 3’s lack of Final Fantasy characters, they shouldn’t give up hope just yet. After all, Square Enix is planning to release story DLC for the game at some point, and it’s possible that the DLC will feature Final Fantasy characters to address fan complaints. However, before fans get too excited, it’s worth pointing out that Square Enix hasn’t released any official details about Kingdom Hearts 3’s DLC or what it will entail, so it’s quite possible it still won’t add any Final Fantasy characters.

And even if Kingdom Hearts 3’s DLC doesn’t have Final Fantasy characters, perhaps Kingdom Hearts 4 will. The Kingdom Hearts 3 secret ending suggests that the game will have The World Ends With You characters at the very least, so maybe it will also let players explore Final Fantasy worlds. But that’s just speculation at this time, and it will likely be quite awhile before we learn more about Kingdom Hearts 4.

Kingdom Hearts 3 is out now for PS4 and Xbox One.

Source: Game Informer