I was thinking about the core problem with Kingdom Hearts. No, I'm not referring to gameplay, story, graphics or anything like that, I mean the actual problem with Kingdom Hearts on a whole. Why is it that some games are better than others? Where is the consistency in content that a KH game should include? The revelation that I came to was quite Nomura-esque and that is simply: it's Kingdom Hearts' fault. Kingdom Hearts itself is the core problem.
This series is completely unique - Disney and Square properties combined. It also isn't a game that is all about cute cameos and no real story, it's also loaded with actual story elements. Can we honestly expect a game that juggles so many different properties to generate a consistent, cohesive plot or have elements that were included in previous installments? Granted, the plot itself exists outside of the Disney worlds, but if 90% of the game takes place in Disney worlds, then how can the main plot really be utilized as best as possible? Let alone, world selection to match the main plot's themes or interaction with Disney characters and the main characters?
If we look all the way back at KH1 - the original and widely accepted best entry - the formula was just right. Final Fantasy served as a guide/home base, the Disney worlds were effectively used filler content that helped beef up the main plot/lore of the series, and the original characters clashed in a great climax that delivered an earned ending. Looking at the next entry, KHCoM, you can also see that the formula was followed. KH2 is where the issues arrive - the formula is thrown off. The Disney worlds are filler with barely any useful content for the main plot, the Final Fantasy characters are no longer your guide (will come back to this later), and the original characters outweigh the rest and did not earn the ending that it received. When I really think about it, KH2's ending only feels earned because of the work that KH1 and KHCoM put into it.
Now, I'm not bashing KH2 here, I'm just saying that it was the beginning of the problem. The formula was messed up. Next we're on to Days (which I adore because of Roxas/Xion, but still) which began the trend of focusing on other protagonists and expanding the story beyond the scope of the ESTABLISHED protagonist - Sora. I'm all for genuine side-games that explore other characters and expand upon information we already know, but Days wasn't just exploring Roxas' time in the Organization. It introduced new plot elements (like Xion). Now the Disney is pure filler, Final Fantasy is absent, and the original characters reign supreme in a story that is heart-wrenching, but also hurting the main story that we've come to know.
After Days is BbS, which I think is the true downfall of the series. We have three brand new protagonists that we've never met or heard of before. The Disney worlds are filler. Final Fantasy is again absent. Now, it's understandable why they were absent in Days, but going to Radiant Garden and not seeing them - after the fact that they lived there was hammered into our heads for years - shows why the formula should have been maintained and why KH2 basically caused this. In KH2, the Final Fantasy characters aren't the guide anymore, it's Sora. Sora knows better than them - so what is their purpose? They've been reduced to a cameo because they can't serve their original purpose. So of course they wouldn't show up in Radiant Garden (their home) in BbS because Terra, Aqua, and even Ven know more than they do. Instead we get more original characters. The worst offense that BbS committed was not being a genuine side-game. Days could have gotten away with Xion, but BbS redefined the Kingdom Hearts narrative and formula. Because BbS - though side-game sized - was treated as a main game, the formula introduced there was carried forward into the next real main game - KH3.
Examining all of that and understanding how we got to KH3, I think we should now ask the question: is this Nomura's fault? Most will giddily yell "yes", but I don't think so. The formula worked, but maybe Nomura had to change it because he couldn't use it the same way anymore? After KH2, Disney became much more successful and also more restrictive in their IPs. It also seems like they became much more aware/interested in the KH series after KH3 was announced (they probably saw the reaction and became cognizant to the one thing they care about - money). If Nomura can't use Disney the way that the formula suggests it should, then what? You increase other aspects? Now that Final Fantasy's role has been reduced (thanks to KH2), do they even need to make an appearance? What would they do?
Ultimately, I see the KH series' problem stemming from KH2. After that, change after change has diluted the series. Don't get me wrong, KH2 was a good entry (albeit a little lacking on the usage of Disney), but it set precedents that ended up altering the face of the series: Final Fantasy characters were no longer Sora's guides, original characters were more abundant and more important than plot, Disney worlds being purely filler, a watered down/pointless tournament shoe-horned into Olympus (so unimportant that there are no secret fights there anymore), and the lack of a plot that builds to that game's ending. KH has lost its true core. Sure, there's Disney and Final Fantasy still, but the execution has been poor. Whose fault is it?