Um, this is a great question. Let me quickly write-up what I think of each series and how much planning they did:
Kingdom Hearts - obviously, as the first one, it is pretty well realized. I will say that, at first glance, it doesn't seem like it hints at any sequels, but it clearly does, with Xemnas' cameo and Roxas' secret ending foreshadowing to II while the end of Kingdom Hearts clearly leading to Chain of Memories pretty wonderfully.
Chain of Memories - I... can't tell with this one. It is clear that several characters were changed in the next one, with Riku being a bit more self-loathing, Axel being waaaaay more compassionate (seriously, he's like a different character in II. Did he have a twin brother or something?), and Namine being a lot more risky (although this last one is more character development than different character). The organization was set up, but I'm not sure if they had the nobody final concept down yet. Maybe that's why this game is so vague as to what these characters are and where they came from?
II - II did some pretty good predicting, in my opinion. It set up the ending of Birth by Sleep, solidified what they wanted Xehanort to eventually he when he was fully realized, and the final mix version brought some significant scenes such as Axel and Roxas' relationship being explored more on the clock tower and the entirety of the Lingering Will, which is still TO THIS DAY one of the best concepts of the series. The Disney worlds were definitely pushed to the wayside and hardly scripted, however. This game didn't feel complete until the final mix came out.
Days - days seemed unfinished by the first mission. The gameplay was straight awful and the inclusion of Xion was clunky at best. As much as I like her, I can't say that anything we were shown before her time even seemed a little like she was a possibility even. Also, that thing where she can wield a keyblade but the keyblade she owns happens to be fake was weird and unnecessary.
Coded - coded feels like an afterthought, through and through. It seems like an attempt to hold people over by remaking CoM and the original while loosely connecting it to a random note in II that certainly didn't feel like it fit.
Birth by Sleep - the ending of birth by sleep seems very well realized, but it was clear that they had to save the best for last in this one, as the stuff building up to it really did not hold any water on its own. This certainly does not feel like it was thought up by Nomura in the grand scheme of things.
Dream Drop Distance - don't even get me started.
0.2 - this one definitely focused on the wrong things, not putting too much thought into why Mickey didn't go back to save Aqua immediately after shutting the door while in the meantime thinking it was necessary to show us why Mickey was shirtless at the end of I.
The X Universe - don't get me started on Back Cover either, AAH. It just annoys me that this game is clearly scripted weekly and has no throughline you can follow. Each time a new "chapter" in the story hits, you can tell Nomura just finished drafting it to go with the story. Literally nothing feels like it was prefaced, there is just too many characters that end up doing jack shit in the grand scheme of things, and it suffers from not telling you anything it's doing, why it's doing it, and when you learn of what it's doing not explaining what it actually is.
Overall, I think X is the worst example of planning, as much as I have a weird, undeserved respect for the series. It is essentially if the pronoun game was made into a video game, HOLY SHIT.
And, don't get me wrong, I hate DDD too, but it seems a little more thought out than X. But only a little.
