AIGHT, LET'S DO THIS.
To start it off, I understand that the length of the worlds remedied why there aren't that many Disney worlds in KH3, but...idk. Even then they just still feel empty. Maybe just...bloat the numbers up a bit. I want the idea of adding at least...a few returning worlds to finish their arcs, completely. I kinda feel there's still room for story in 2 worlds - Agrabah & Neverland for their respective sequels (or Pan's sequel & Aladdin's threequel)
Next possibly the inclusion of 2 new Disney worlds. Atlantis (Atlantis - the Lost Empire) & the Crescent City (The Princess & the Frog). Both of which introduce 2 of the plot points - 2 candidates of the new 7 Lights, Tiana & Kaida, and Atlantis would be a perfect place to have some sort of connection between the Atlantean War & the Keyblade War.
SOMETHING To ACTUALLY involve SDG in Arendelle. Idfk confront Elsa, tend to the actual town, ANYTHING.
This probably goes for the Caribbean. Have Sora more involved in the shit that's happening in regards to Davy Jones, Beckett, the Brethren Court, Sao Feng...honestly if Beckett was more involved here the same as he was in At World's End, I might actually be reminded of his existence. And in all honesty, just...have everyone go here after the chest. Have Maleficent & Pete ally with Beckett & Davy Jones to make things interesting.
The Pixar worlds have done a MAGNIFICENT job in making it FEEL like the Disney worlds seem actually...relevant. At the expense of a non-fightable Randall or Zurg. So perhaps just have them tag team with the Heartless you fight. Like Zurg, discarded and brought back by Young Xehanort riding alongside the King of Toys, and either Randall, Vanitas, or both fighting Mike, Sully, & SDG alongside the Lump of Horror.
I've Seen the Light at the lantern scene in Corona. We got Let it Go AND Do You Wanna Build a Snowman, I don't get why we didn't get that song along with them.
Lastly San Fransokyo. At least...
SOME sort of connection from Donald & Goofy in regards to the Darkubes & the Blox from Re:Coded. I don't think we even got a single reference to them. Also, another opportunity to include Maleficent & Pete into the world instead of just....inserting them there just to remind us that they exist. Maybe the Darkubes can pique Maleficent's interest, given they look similar to the blox in the datascape. Perhaps Maleficent can conclude that they're the key to the mystery behind the data world she needs.
And
THOSE Are just the Disney aspect. Literally the ONE thing I could change in regards to the original story is just...scatter the story arcs around. Like...have Aqua & Ven's rescue earlier in the game instead of shoving it ALL at the end of the Disney rollercoaster ride. I'd say get to this part of the story after say...the 2nd wave of worlds. (In this change it seems to be...1st wave - returning worlds, 2nd wave - new worlds.)
LASTLy, an IMPORTANT THING, HERE. 3 lacked something that 1, and to an extent, 2 has. A MAIN world base...if that makes sense. A world in which the group goes back to, to either restock on potions & equipment or reconvene with the main and Final Fantasy characters. KH1 has this with Traverse Town, and 2 has this (to an extent) with Hollow Bastion. 3 had Twilight Town, but it didn't feel like it had any incentive....go there a 2nd time, let alone more than once. I mean...there's doing 100 Acre Wood and all, but idk.
That being said, as much as I like 3's Twilight Town, Radiant Garden fits this category the best. The exploration of all the areas we've been through of this world, throughout the series. The lush castle gardens and the very town itself of BBS, the outskirts of the town & the Great Maw of 2, and the castle itself of 1(With...less platforming). You can reunite with everyone in either both the castle and Merlin's house, with the FF cast, and Ienzo, Aleus, and Dilan. It'd be the perfect 'hub world' of KH3.
I've thought way too much of this. Some points probably don't make sense. But as we're just one (or 2 I guess) games away, I enjoyed participating in this. Even if my points are probably all just...gibberish.