This has been a key part of how I think the series should go. In fact, KH has written itself into such a narrative crater that there isn't much scope for anything else if you want to tell a good story.
That, paired with the loss of storytelling radicalism marking KH1, CoM and KH2, means I'm not personally optimistic they'll be able to climb out.
Melody of Memory did not instil confidence that it marked a fresh start. It just doubled down on the ridiculous pseudo-lorebuilding gobble gobble that mistakes breadth for depth. KH's story used to sound ridiculous if you just summarized it reductively out of context. Now it's ridiculous even with context.
Riku's character has become redundant following the conclusion of his arc. At this rate, the rest of the cast will follow his lead very soon.
In my personal "how I'd salvage the series if I was in charge" headcannon, Riku gets stuck somehow while searching for Sora in this new reality. Whether that's stuck temporally or spatially, I don't know. Possibly both.
He begins to stew in his own actions from earlier in the series. Eventually going way overboard on the "light" to battle his overwhelming guilt toward the Teen Angst Edgelord Emo Riku phase.
Like many religious converts IRL, he becomes evermore puritanical and radical. Like many religious converts IRL, the radicalisation stems from displacement, upheaval, and the shattering of one's worldview. Riku fits that bill perfectly.
The tragedy would be how the state of balance he strived to reach– for like 10 games in this series– falls apart once again. From doing "bad" things for "good" to doing "good" things for "bad". The centre cannot hold. Literally. Riku's flaw has always been proactive maximalism as an instinctual response to problems. Ends first. Means later. There's echos of this in KH3 but the fact he's completed his arc means that his opportunistic ruthlessness is gone and he's become a reactionary lost puppy. It's boring. It's redundant. All the complexity is absent.
"Priest Riku" being the series' final main villain, it brings Kingdom Hearts, Riku's arc, and Sora's journey full-circle. Riku wants to return every disconnected world to that mythical "one world under one sky" monolith, whatever the costs. Sora, meanwhile, grows out of his wistfully simplistic view of morality (perhaps too far in the other direction) and seeks to preserve world order – for better or for worse.