Hi, I'm new, so don't mind if I join you, I hope!
TL;DR: I don't think it's a big deal to continue to wean off FF characters, since Sora, by now, is more grown and able to wield much greater power. But, at least a cameo of all of the FF characters would be nice.
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My explanation:
In terms of the rules for marketing games and anime in Japan, you should usually "solve every problem" in sequels that follow - therefore, should that require less help from supporting characters, it should be done to wean them off to avoid creating too many more issues in the series.
Much of the KH series has been about solving different problems, and leaving the main problem "Xehanort" to the main series. Using FF characters too much would cloud (no pun intended) Sora's judgment and dexterity in problem solving, and make things too hairy for the situation.
Let's not forget, they (the main characters) are not "kids" anymore, they are grown and ready to tackle things themselves. By now, Sora's powers are great, he should be able to manage.
Obviously, they want to send it out in the best time to make as many sales as possible, and keep up with the proper order of release schedules of Square Enix games. Shall we review them? MAJOR: Dragon Quest XI: Echoes of an Elusive Age - September 4, 2018 MAJOR: Shadow of the Tomb Raider - September 14, 2018 MAJOR: Just Cause 4 - December 4, 2018 Then... MAJOR: Kingdom Hearts III - January 29, 2019 --------------------- Since they cannot release the game before September, and doing it in October, November, or December will crunch it in with the holidays too much - January was next best time to launch.