I also would prefer the Foreteller Saga to not end up as long as the Seeker Saga, 17 years for one Saga is a bit much. I'd like for it to begin and end in this decade.
It'd be nice, if the saga and games were better planned out. Maybe that Nomura would consulting the Light Novel and Manga authors a bit, seeing I feel Kanemaki and Amano do a great job with his ideas and often tell it better than he does.
Other things I'd like to see are the worlds to become more explorable, with secrets that reveal themselves as you gain new abilities. All while keeping the size of KH3 worlds.
For the treatment of female characters to improve, I have serious issues with how KH3 treated most of the Girls. Though the game did make most incompetent that weren't Sora. Also moving forward it'd be nice, if when new characters are introduced there'd be more girls/women among them.
I'd like to see them also find a way to handle the large cast. Maybe spinning them off into their own lines of games?
Also I think it's time we'd get a game staring and developing Kairi. She's been sidelined for too long.
Oh and I'd also love to see minor side quests become a thing, it'd be a great way to explore and flesh out the worlds and story that aren't the main plot.
Since it was first introduced in KH3 and what it represents, I may also say I would love Verum Rex to become a real game and it's own IP. I say it's own IP, because I want it as far as possible from Disney claws. Just like Final Fantasy and The World Ends With You franchises, it could still crossover with KH in the future.
Well considering Verum Rex first appeared in Kingdom Hearts, I wouldn't be surprised if the IP now technically would belong to Disney, seeing all KH original Characters are Disney owned to my knowledge.
Obviously there could be some special deal in place that would except Verum Rex and It's characters from this rule.