Nomura would need to rediscover, or reconfigure, his theory of the narrative. The ship has sailed (or sunken) on Xehanort's saga as a whole, and trying to recall its specific plot points in order to clarify the trajectory of an unfolding story would be an exercise in futility, not only because it would further muddy the waters by intersecting ideas which Nomura hasn't made any meaningful effort to connect, but also because it misses the point by a significant margin: KH has always been vague and noncommittal with regards to its utilitarian elements (i.e. How Shit Works). That's not what sank KH3.
The problem that "broke" KH3 and which presents an issue for the series moving forward is that Nomura either had no idea what story he was writing or had no clear and confident method of communicating that story using the parts he had assembled for himself over 10+years of narrative development. It wasn't that he revised or retconned character and story arcs through the introduction of new elements that are not carefully explained, he's been doing that forever by accident or deliberation; it's that he chose to apply such broad strokes in writing his characters through that process that none of it feels essential to them and it thus hampers or obscures (or eliminates) their relationship to the larger work. Aqua and Kairi get placed in plot freezers with no conceivable benefit for them as characters; Axel has his conflict with Saix regressed to a dispute not of personality, nor of will, nor even of moral direction, but of an attention deficit that speaks more to a lack of believable friendship between them rather than a breakdown in something meaningful; the ensemble is altogether mismanaged to the extent that nobody has any particular reason to be anywhere for most of the story, driven neither by a pursuit of one another or some greater objective which could expand the scope of ideas and possibilities fueling this ostensibly climactic chapter, but by the smallest form of narrative pragmatism: because the damn thing has to end sometime. Characters convene and are separated according to a logic of rote convenience: the story as it has been thinly laid is just more manageable if things unfold in a way that doesn't actively coordinate the best possible telling of it with the outcomes that have been earned over the course of the series. It's a simple trade-off: figure out a way to represent the whole tenor of Aqua's personal journey and give closure to her struggles by bringing her heroism and humanity into focus, or shove her off stage for most of the action because allowing her to participate in the story creates "problems" that belie the corner into which most of this story's key factors have been written.
This is not the issue of a functioning narrative beset by faulty turns (again, we've had those KH games before, like hi II and BBS), but of a dysfunctional narrative aiming to fill in its faults with incoherent material that doesn't effectively cover for the lack of structural integrity, and in many cases only embellishes the sense of disaster. This is a game that was marketed on a thematic notion of "resolution," only to make the most technical, literal endeavors to bring its legacy full circle with all the creative spark of drawing up a grocery list while using narrative recall as a blunt instrument to justify (or distract from) a conferment of all of its real dramatic energy upon a set of narrative concepts lifted from another installment that have yet to coalesce into something resembling an actual story: one can only imagine why it comes off feeling less than fully formed. The characters in KH3 and their corresponding stories lose definition and thematic immediacy as an expense paid to that design: some of them have none to begin with, because their presence in KH3 is entirely predicated upon their future relevance (hi Larxene, Marluxia, Luxord, Demyx, oh wait did I just list half the antagonists?). KH3 is a sham of a resolution to anything, but Nomura still opted to propose that as its core conceptual drive, which indicates either a deep wanting for self-awareness as a writer or an erroneous way of articulating his own creative process, and either one is enough to "break" a story that requires the level of micromanaging and obsessive attention to what is not known as KH. It's easy to lose the forest for the trees with this series, and Nomura appears to have compounded the problem by bifurcating the best laid route in an effort to get where he wants to go faster than his own narrative would have allowed. A continuance of this tendency will only result in a story that feels increasingly pared down and directionless, as well as repetitive: people who write without aim tend to write in circles.
I guess the only question I would ultimately propose is what keeps Nomura creatively tethered to this series: what does he want to do with it? I don't know that there's ever been a consensus surrounding that within the fandom, but that wasn't really a problem because between I & II, and I would argue even moreso in the handheld era, I think there was a general confidence that Nomura himself had a vision, for better or for worse. But now between KH3, the endless gacha game standbys and the (likely) return to a SoRiku duopoly on meaty drama, there's a sense that wheels are being spun, and the output feels not only incomprehensible on an intellectual, left-brain level, but also increasingly seems to have lost focus of the personal and emotionally resonant rationales behind the series' conceit. I guess that's also more generally an issue with media consumption and franchise storytelling now: having a destination isn't as vital an expectation as maintaining a continual flow of cash. KH3 and Union Casino do that so...what is it that we're proposing to "fix," again?