I could say many things. But I think what KH is missing most is friendship. Literally. (kirabook here from the future. I ended up writing it all anyway. I'm so dumb)
When I think about KH1, I think of the legit friends I feel like Sora made. Ariel. Jack. Aladdin. Hercules. Tron. The FF crew. Those especially stick out. They felt genuine, like Sora was really connecting in these places and making lifetime pals. It contributed to this connection when other characters in the worlds knew he wasn't from there, and not always the villains (King Triton. Small as it may have been, Sora getting chewed out for being a literal outsider sticks in my brain)
It wasn't just Disney characters either. Even though Riku and Kairi's screen time was a little sparse (cause mystery and intrigue), I felt like Sora was really trying to find them. Every time they interacted on screen felt like a big moment, if that makes sense. Like "Phew. They're back together again however short lived."
Although I liked the story of CoM in the in between parts, CoM always annoyed me a little with the extremely useless Disney World revisits. I felt nothing experiencing those worlds and characters again (... except the FF crew, theirs felt meaningful). But I really enjoyed seeing Sora get so upset and confused as his memories got more twisted. And though Namine's existence and powers confused me, I felt for her, being used by the bad guys to hurt someone she cared about.
KH2, while I have my issues with how they presented some of the plot, still managed to keep Sora's friendships with these Disney characters important and relevant to me. It truly felt like Sora was going to visit his friends and hang out. Sora meeting the FF crew again was like their dopey little brother finally coming home after ignoring their calls for a few days. Once again, although Riku and Kairi's time was sparse, it still felt like they were truly friends trying to make things work. KH2 did great to keep the trio thing going even though there could've been more.
I think Days wins over a lot of KH fans because despite the lack of "Disney magic", the game thrives from character interactions sorely missing from future titles.
Birth by Sleep, as much as I love it, could've done more with the trio love and Disney world situation. The only last friendships I can really think of is Cinderella. And Zack I guess. If Sora uses Disney worlds to hang out with da bois, the TAV uses Disney worlds for formal business meetings no chillin' allowed.
DDD has.... nothing. Absolutely nothing. No interactions between Sora and Riku. The Disney worlds/characters aren't real. The most you can take away is TWEWY and even that is lack luster. The whole SRK "trio", at this point, is barely a trio as they missed yet another opportunity to have Kairi train as well. It's a whole mess. The most memorable part of this entire game is Sora having an acid trip, interacting with Roxas, being rescued by Ven's heart, Mickey and Riku failing to rescue Sora, Axel rescuing Sora, and Riku rescuing Sora. So basically, the final 1/4th of the game which is entirely composed of OC interactions (at this point, I'm counting Mickey and pals as OCs) (kirabook from the future. I FORGOT ABOUT TRON, I LIKED THAT A LOT. Only the Tron part, the rest of that world was bland and pretty useless. Not to mention no Tron payoff or mention at all in future titles)
0.2 is an odd one. I'm probably extremely biased since Aqua is my fav. Or maybe it's because though they were only briefly around (Terra, Ven, and Mickey) it almost resembled Sora's KH1 journey??? Like every time they appeared it was a big character/relationship building moment. Maybe it's because we were revisiting Aqua's twisted memories and people who missed her arc before can now see it plainly written that she kinda hates herself and sees herself as unworthy half the time. Like a true character study, something we rarely get to see. Something kinda like Sora in CoM. Something like Riku's journey we only saw bits and pieces of.
Then we come to KH3. It has a similar issues as DDD for me. What... connections are there??? What friendships did Sora form that I think will actually last? I mean Hercules made a return and it was pretty cute. That summon with Ariel is absolutely adorable. But do I feel like Sora truly connected with Rapunzel and Flynn? Not really, acquaintances at best? The monster crew? I mean, Yeetus Vanitas is pretty memorable but I dunno. Toy Story? ..... I guessssss. Moreso than the others so far. Definitely NOT Frozen. They probably don't even remember Sora's name. Big Hero 6? Eh...... It's all so sketchy. The Twilight gang! That's right! They were there. I suppose that friendship/connection is maintained.
Not enough of the new KH content properly allows Sora to be a part of it. These new relationships he's building don't feel real. Also, pretty much no one else (OCs) is interacting with the Disney worlds at all EXCEPT Sora.
To recapture the magic, they should consider reintegrating all these characters back into the mix of OC and Disney. They should maintain the relationships OC characters have formed with Disney characters. When new worlds are introduced, they should do better to make Sora's relationship with them more natural and believable. And lastly, the OC characters need to interact WAY more. Nomura spent so much time and effort connecting all these people together. Saying how desperately they want to meet each other and be friends. And yet when they're in the same room, they're just staring at each other like "
Nice face. Now moving on to Xehanort."
See? It all boils down to friendship. I'm not talking about the sparkly word version of friendship. But actual screentime on screen together talking to each other, hugging each other, punching each other, anything other than stand 6 feet apart since the only corona in KH is the Kingdom of Corona.