Was it???
Some of the secret reports, which are postgame only, are very reliant on it. But the main story??? Everything from the X time period that appeared in KH3 was covered in X Back Cover, which was part of the collection of "things that lead up to kh3", as well as something players would actually be expected to watch like every other full KH work (i.e. not cutscenes of an abusive gacha game). In fact, most of it was only in X Back Cover and not really explained in X/UX at all. Meanwhile Subject X's connection to any of this stuff is not established until the secret reports, and on its own is an answer to a long-unresolved xehanort saga plot point.
I don't get how these are unresolved. What is there to "resolve" about nobodies? They're normal people, this was thoroughly explained. Xehanort explained how he time traveled in the first game it was introduced, and now he's dead. Keyblades... are the central symbol of the entire series so I think "resolving" keyblades would end the series completely.
The whole Girl X storyline - which basically watered down Axel/Saix's storyline from what we knew by forcing in that character (highly likely to be Skuld). I like that this KHUX character was added into their storyline, but I don't think THIS was the game to add her into it. Instead of building on Axel abandoning Saix for Roxas/Xion, they added in Girl X as well which didn't do anything to enhance that plot, but build to the future (and take away from the current plot).
Nobodies being a thing opens up a can of worms for future installments. So does time travel. I'd assume any keyblade wielder had a "strong heart", so if they lose their heart then the question floating over that incident will always be "where is their nobody"? Time travel being so messy in the series also opens up the question: why doesn't Character X just time travel?
Keyblades need more explaining like where does Mickey "power up" his keyblade, where did Kairi get hers from, where did Riku get his new one from, what is handing out keyblades to people? One story element that I actually think could be intriguing and build upon KHUX's plot of all the keyblade wielders dying out and essentially becoming extinct would actually be the trend of keyblades being handed out in recent games. It could be whatever the source of keyblades trying to repopulate the worlds with keyblade wielders now that Xehanort is gone. I think there's a story there that could be told, but Nomura never goes deeper into the nature of keyblades or why there are periods of time in the series with LOTS of wielders (KHUX) and barely any (KHBbS to present).
Dudes be like "Here's what SHOULD happen in Kingdom Hearts: (posts fanfic)"
I am going to be so genuinely angry and mad and embarrassed if someone finds an example of me doing this years ago to own me
Where the hell did I say that this SHOULD happen in KH? I said that I think the KH series would do well with a reset to help usher in the new saga without the baggage from the old one. We already saw the problems KH3 had trying to deal with its own loose ends, so what would make KH4's storyline more streamlined with all this stuff from the Dark Seeker Saga still unresolved?
Honestly, this type of response is why no one is posting on here. Back in the day, people actually were willing to post theories and engage the community with ideas. But nowadays, there are people like you that will condemn and shame people for suggesting ideas/theories and gatekeeping what KH should be. You disagree about a reset in the series, that's fine, but don't make some sly comment basically saying that it's embarrassing that I posted my idea.
Maybe you don't stand by what you post and would feel embarrassed by something you posted years ago, but I will stand by my post regardless of whether you think it's embarrassing or not that I came up with an idea for discussion. Obviously what I posted isn't exactly what I would do if I were in control of KH, let alone I might not even do it at all if I were in control of KH. The point of the post was a discussion on whether people think a reset is a good idea and I gave an example I whipped up in like 5 minutes to create a visual to go along with it.
But thank you for your comment and adding to the discussion.
Or they could just have an alternate universe that is its own canon. No need to do some weird reset thing and have only Sora, Aqua, Kairi, and Mickey (why you chose these four out of everybody else, I do not understand).
An alternate universe with new characters would be the best refresh for the series if you are talking about "resetting" things.
I chose them because I can't see them without keyblades (not sure what weapon any of them would use). But anyway, I like the idea of an alternate universe with fresh stories - which is what Quadratum could be. However, I feel like Nomura would be too tempted to cross-over and connect them.
The main reason why I think a reset is necessary is because there is way too much to explain away now. If the next game happens and there's a huge portion of the cast missing, logically everyone would ask "why didn't they help?" Nomura went out of his way to make characters locked away, sleeping, possessed etc. in the Dark Seeker Saga to explain their absence, but now that they're all freed, now what? Is he always going to have to use at least 10 characters (Sora, Riku, Kairi, Aqua, Terra, Ven, Roxas, Axel, Xion, and Mickey) every story? Or will he have to explain them away every time?