For me personally, it's a toss-up between several options:
1. Better dialogue. I know the silliness of the writing is one of the things that gives the series it's charm, and we all like to make fun of it, but for real, I would like to see some genuinely good dialogue every once in a while. Not there isn't some good writing here and there, but it's far and few between all the light heart darkness heart. My favorite part of the series was the opening of KH2 when you play as Roxas, and that's primarily because there was so little of that bullshit. For a while, it was just a simple story about a kid losing control of the world around him and losing his friends. There was very little of the traditional Kingdom Hearts word salad—in fact, most of the dialogue, with some exceptions, was refreshingly down-to-earth. I'd like to see more of THAT dialogue come back.
2. More flashback cutscenes. So much of the lore of Kingdom Hearts is simply told to us through in-game dialogue or secret reports. When you stop to actually consider the events being related, the story of Kingdom Hearts is actually amazingly vast in its scope. But because you see very little of it, the games do feel a little more spatially constricted. For example, we were told in KH1 how Maleficent took over Hollow Bastion 9 years before the events of the game. Wouldn't that have been amazing to actually SEE? (and hopefully, we might actually get to see it finally in Melody of Memory). There are countless examples like this throughout the series that would make the story so much more visually engaging, and easier to follow. I really do believe that at least half of the confusion people feel when playing these games is because SO MUCH IMPORTANT STUFF is never shown to us, and either glossed over briefly in character dialogue, or, much worse, in supplementary materials like the Ultimania. I realize that back in the day, there was probably more restrictions on what the PS2 could handle on a technical level, and they probably couldn't have shown something so large-scale. But now that we're in the PS4-going-into-PS5 era, there's little excuse for that anymore, and I wanna see me more flashbacks.
3. More Final Fantasy characters, AND I want them to be more important. The Radiant Garden committee seems to be the only FF characters in the series with any modicum of importance to the overall narrative. I don't want some measly cameo like poor Zack, I want actual goddamn involvement.
4. Either less focus on Disney worlds, OR make them more relevant to the narrative. Now, unlike most people, I actually believe KH3 did Disney worlds even BETTER than KH2. Frankly, it was the most worthwhile I've seen the worlds since KH1, and it was really refreshing. But it could be better. KH has never achieved its full potential in regards to utilizing its properties, and I want to start seeing more Disney characters besides Mickey, Donald, Goofy, Yen Sid, Maleficent, Pete, Chip, Dale, and Jiminy Cricket touring the worlds with our heroes. I also want to see the villains have more involvement, too. Now that the Xehanort saga is over, and especially now that Nomura has proved to the suits at Disney how financially successful and popular this series can be, hopefully Nomura can sweet talk them into being a bit more loose with their properties so we can get some actual goddamn crossover action for once.
Those are my four biggies.