You make a really good point. "Kingdom Hearts 3" legitimately could have been all brand new stuff and a side game could have cleared up most of the side stories, and I think that would have been fine. The reason I say that is if you follow KH1 and KH2, neither game had previous games that actually "built up" to them. You didn't need to play CoM to understand KH2. In fact, KH2 goes out of its way to ignore/brush aside the events of CoM - to the point where it's cool if you did play CoM, but you didn't then it's not really important to know to advance the plot.
Really thinking about it, if they expanded 0.2 a bit more they could have covered:
- Riku and Mickey saving Aqua
- Aqua saving Ven
- Kairi/Axel training
- Explain the Replica Program/time travel more
That would have opened up Kingdom Hearts 3 to be solely about the Keyblade War and the aftermath of Master Xehanort's plans. Now that I'm thinking about it, a really out there idea for Kingdom Hearts 3's plot:
- Sora visits Olympus and regains the Power of Waking (by being a "hero" and going to Mount Olympus and doing some kind of trial).
- Since all the lights are assembled: Sora, Riku, Kairi, Axel, Mickey, Ven, and Aqua they proceed to the Keyblade Graveyard and initiate the Keyblade War.
- After they save Terra and Roxas/Xion return: you fight Master Xehanort, but Sora has to use the Power of Waking to break time again. This causes him to end up in Quadratum.
- Sora has to find his way out of Quadratum - he visits Star Wars and the alternate Toy Box (which makes him realize that he is in fiction and the world is split, so if he does something on this side, the other side will see it). He does that to get a message to the other Toy Box world.
- While exploring DISNEY worlds to find clues on Sora, one group finds the clue from Sora in Toy Box - which leads them to ask the Fairy Godmother for help to access dreams or whatever.
- Yozora is following Sora throughout the story and is the second to last boss - the Hollow Bastion Riku boss fight - in order to escape Quadratum.
- When Sora gets back to the real world, Master Xehanort has the X-Blade and is going to use Kingdom Hearts - but everyone has to team up against him and they finally kill him.
- In a secret video, we see that Yozora goes back to the Foretellers and give details about a keyblade wielder that was in Quadratum - this makes them realize they need to go back to the real world.
This needs more work to iron out the details, but I think it would have been cool to swerve away from the Keyblade War and go in a completely different direction for most of the game, then come back to the war at the end. Also, I think this would have been a cool way to allow you to use different characters. I imagine it would be Sora and Strelitzia in Quadratum (with the DISNEY world party member), then in the real world segments you could choose between Riku, Kairi, Roxas, or Aqua.
I whole heartedly disagree on the usage of the DISNEY worlds in KH3. I think it's the best use of DISNEY worlds since KH1. Not only are the worlds really fun to explore and go back to giving each one a distinct feeling while exploring them (Olympus is climbing from the town to the top of the mountain, Monstropolis is a boxed in warehouse, The Caribbean is an open sea to explore via boat, San Fransokyo is like an open world superhero game), the DISNEY worlds actually impacted Sora and the main plot.
Olympus introduced the "black box" storyline, had Sora learn about strength and sacrifice.
Toy Box introduced Yozora and was used as research to perfect the Replica Program.
Monstropolis brought back Vanitas (probably the best use of a DISNEY world ever).
San Fransokyo was used as research for the Replica Program and showed Sora how a heart can live on without a body.
Arendelle revealed 2 Princesses of Heart (increases the stakes for Sora), and taught Sora about sacrificing himself.
Kingdom of Corona revealed a Princess of Heart (increased stakes for Sora), and taught Sora about sacrificing himself.
The Caribbean expanded on the "black box" story and taught Sora about sacrificing himself.
I know that it was probably missed, but the DISNEY worlds were basically either explaining how the Replicas were fixed (at least that the Organization was still working on it) and building up to Sora's ultimate sacrifice. I liked that Sora saw several examples of the "DISNEY happy ending" after the sacrifices EXCEPT for The Caribbean which is smartly placed as one of the last worlds before the Keyblade Graveyard. I think it really showed Sora that there could be some actual consequence to sacrifice and that love might not be able to defeat all.
Which then ties in to Sora sacrificing everything to take the forbidden path to save everyone - especially Kairi. I think the line from The Caribbean about "one day may be long enough" really stands out when you think about the ReMIND ending with Kairi. Anyway, I'm just saying that I experienced the DISNEY worlds in a different way when I played them. I definitely do think that there is room for improvement, but I disagree when people say the DISNEY worlds were just an afterthought. I actually think KH2 is the biggest offender of that - outside of Beast's Castle, Space Paranoids, and The Underworld (a stretch) - none of the DISNEY worlds mattered. On top of that, their designs were - for the most part - really bad. Just giant boxes with a DISNEY themed wallpaper.