Kingdom Hearts supposedly ended its Dark Seeker Saga with Kingdom Hearts 3, but the Dark Seeker himself, Xehanort, is still sticking closely to the series. Xehanort is the main character of Dark Road, a side story built into Union X, and the story revolves around his youthful days as an apprentice. The story ultimately raised more questions than answers, and at this rate, it may be for the best if Xehanort gets left behind altogether in future games.
Union X characters, namely the Foretellers and the Master of Masters, look to be taking center stage in the story from this point. Bringing back Xehanort would bloat the story even more than it already has been. For all of the convoluted plot turns, Kingdom Hearts 3 did close off that plot thread, but there's one main problem, though: Young Xehanort.
Xehanort's Inconsistent Lore
One of Kingdom Hearts' most infamous plot points is the abundance of Xehanort variants. Ansem (Seeker of Darkness), Xemnas, Old Master Xehanort, and Young Xehanort make up a team of separate people all formed from the same person. Each new variant made the Xehanort story more complex, but it was followable.
Xehanort, a native of the Destiny Islands, felt stifled by the tiny world and left to explore other worlds. He became a Keyblade master and eventually became swayed by the darkness. Once he reached old age, he stole Terra's body as a vessel. After being defeated by Aqua, Xehanort woke up as an amnesiac in Radiant Garden and became an apprentice to Ansem the Wise. He eventually removes his own heart and those of the other apprentices, forming the Nobody Xemnas and the Heartless Ansem.
Dream Drop Distance threw a wrench into that storyline with the introduction of time travel, and Young Xehanort as a separate character. Kingdom Hearts 3 introduces what appears to be an even younger version of Xehanort, playing chess with Eraqus in their apprentice days. "Chessnort," as he's sometimes nicknamed, makes little sense chronologically. He appears to be the youngest version, but has already left the Destiny Islands. Add in the Master of Masters' cryptic conversation with Young Xehanort in Re:Mind, and things start to get frayed.
The Future of Kingdom Hearts
Kingdom Hearts has a problem where when more is revealed, less makes sense. This is especially apparent with Xehanort's narrative. The games have hinted that they're not done with the old keyblade master just yet, but it may be for the best if they just drop him altogether. Enough was done to make the end of the saga finite, and there's no need to drag it on any longer.
The Foretellers and the Master of Masters have enough story to explore on their own, and they don't necessarily need to be connected heavily to Xehanort or other major characters beyond what's already been revealed. Quadratum is another mystery that's set to be a major feature in future titles, and that will have its own complex narratives to follow. Now that Xehanort's story is ostensibly over, it should be kept that way.
Kingdom Hearts 4 is not known to be in development.