I felt DDD did a really good job incorporating the disney plots into the main plot as well. A Xehanort appeared in every Disney world and they were the ones guiding Sora to those worlds because they wanted to "teach" him things.
-Taking Sora to La Cite Des Cloches to show him someone who made their heart a prison by introducing him to Quasi, the scold him for being a hypocrite for trying to give Quasi advice on the matter.
-Taking Sora to Prankster's Paradise to use Pinocchio to teach him about empty vessels growing hearts.
-Taking Sora to The Grid to basically enjoy his pain in having to watch his friend Tron (well a version of him anyway) be enslaved with his memories manipulated, using it as a way to explain how hearts are formed and how data differs, and then use that as a way to dissect Sora's character bit by bit to make him question if the bytes (his memories and emotions) that make him up are truly all his.
-Country of the Musketeers is a world Sora took himself to and while it didn't effect DDD's plot, it did affect the overall series plot because while Sora and Riku are training/taking the test the game is also running a side story to expand on Mickey's past so they encounter Mickey in different points of his own training to be a master. This world also shows us how Mickey and Minnie met, presumably how the boat boy became a king, and Pete's first known act of real villainy. Since the english version of KH1 onward had Donald and Goofy using "all for one and one for all" this is just sort of a happy accident that it sort of retroactively connects their use of that phrase to this world.
-Symphony of Sorcery is another world Sora took himself to, to take care of the subplot of Spellican who had Sora was chasing to defeat in order to help the TWEWY cast. Young Xehanort makes a surprise appearance to clarify to Sora that the norts are not a dream or part of his test, and that everything was coming to an end. We also get a rare moment of character we get to see a Xehanort show genuine appreciation for the natural beauty and wonder of a world....the fact he crushes a petal in the wind is also actually important to because it subtly connect to something data Ansem the Wise says later on "the petals in the wind, there are hearts all around us and it doesn't take superpowers to see them" because then basically it means in this scene it highlights even as a kid Xehanort wasn't capable of seeing the hearts that surround him and when he crushed that tiny petal if it was real he would of been crushing a tiny little heart. Oh also it's kind of a fake out, because Yensid's tower is suppose to where we appear when the quest is over so getting this fake out and then the pull a DOUBLE fake out when you think "okay now for the real tower" only to pop into TWTNW.
Riku's story wasn't so connected because the Xehanorts were mostly ignoring him but even then every Disney world for the most part still felt meaningful.
-La Cite Des Cloches he runs into Frollo and has to face a person who in their obsession had fallen so deep into darkness he can't be reasoned with, like Riku once was. He also gets to meet AnsemSoD who basically exists to remind him of that and get Riku thinking about the things he keeps locked inside his heart he is trying to figure out what to do. Oh and of course his whole speech to Quasi "Go out there and get some real friends, trust me a true friend will be able to see you for who you are no matter what you look like" a beautiful showing of the lesson he learned back in KH2.
-The Grid he basically meets a guy, Flynn, who is very much like Xehanort. It makes Riku muse over how Flynn's single minded desire to find some greater answer caused him to lose sight of the danger and created CLU, is the same as how Xehanort lost himself in obsession and created Ansem...and how easy it is for anyone to create an "Ansem" of their gown.
-Prankster's Paradise probably the most important world in Riku's growth because he is forced back into a record of arguably his most shameful and cold moment, back when he scoffed at Sora's talk of a "conscience". Here he gets to make it right and help Gepetto by bringing back Pinocchio and then upon talking to Jimminy he muses over that time and Sora. He realizes Sora is basically his conscience, his friend with the stupid grin who shows him right from wrong, the best teacher he could ever ask for in this department. He also is mildly shaken by seeing himself taking Pinocchio again and it forces him to reflect on how his single mistake of giving into the darkness in the past has caused him to be chased around by dark phantoms since then be it Ansem's shadow, a replica of his dark side, or literally himself.
-Country of the Musketeers while Sora's story is expanding on MDG Riku's is expanding on Minnie, it's Riku being called a hero/musketeer by Minnie which is the kind of praise Riku never really gets so that's actually pretty meaningful. It also ties into the big mystery of Riku's story, starting from the second traverse town visit the plot had shifted focus there being two worlds. This world is the first time where we the player find that Riku's actions in one world can still bleed over and affect Sora's via him stopping that trap.
-Symphony of Sorcery a lot of the same stuff as Sora except he's not here because of spellican, his story mostly focuses on giving us more Mickey Riku friendship bro time (which is nice since they have a surprisingly small amount of times they've actually talked to one another) and another evidence of Riku's actions affecting Sora's world and vise versa. Oh and it's the second time a nort appears in his story, Young Xehanort who draws him down in the dark music area, tries one last time to shake Riku, and after his failure to do so he cryptically hints that Riku will fall into the abyss soon and leaves. (which Riku ultimately does do, but Riku was drawn in with Sora so he was able to still fight his way out unlike Sora whose falling asleep caused them to drop)
I really liked the Disney World involvement in DDD, it's so rare we have Xehanort appear and actually interact or talk to/about the Disney characters and this game gave us multiple worlds of that.