Overall, Kingdom Hearts 2 is my favorite game so far; largely because of the overall size, scope and length of the game. KH2 has more worlds than any of the other games and I love how most of the worlds have at least 2 story missions, adding to the overall content and length of the game. I really hope that KH3 has just as many, if not more worlds and missions.
I love the idea of being able to play as other characters besides Sora in KH3, namely Riku, Kairi and Axel/Lea. I think there should be a part at the start of the game, where you start by playing as someone other than Sora, kind of like how you play as Roxas at the beginning of KH2. Maybe the game could start with Aqua, who has escaped from the Realm of Darkness, or is in the process of doing so and hopes to contact Sora and ask him for help.
I don't really like the whole reaction command thing, though and how that and almost everything else in KH2 is so tightly tied to the triangle button. I think this was a gross oversimplification and I don't want to see the same mistake repeated in KH3. A gameplay feature that was introduced in KH2 that I WOULD like to see return in KH3 is drive forms. Drive forms were probably the best, most awesome part of of the KH2 gameplay and I really want to see them again in KH3. Maybe some new ones? I don't really like the look of this Disney park attractions mechanic that they showed us in the gameplay trailer. It looks interesting, but also a little silly and I hope it isn't featured too prominently in KH3.
As for movies and worlds, well there's one world in particular from KH2 that I REALLY REALLY REALLY want to see return in KH3 and that's The Pride Lands. I, like so many, love The Lion King and I think it'd be great to see the Lion King 2 story used in KH3 and to see the Pride Lands restored green and beautiful again. I love visiting The Pride Lands in KH2 and seeing Sora as a cute little lion cub and the cool, unique play style that comes with that form.
I also think that Sora should gain the ability, later in the game, to change into his lion form at any time, anywhere, just like a drive form, or a limit. This would be really awesome and when you consider the fact that the early character concept design for Sora was that of a human/lion hybrid, it just seems fitting for him to have this power. Actually, I think I'm on to something here. What if other characters, like Riku and Kairi also had some sort of creature transformations? I could see Riku turning into some sort of gargoyle, or bat type creature. This would fit with his darker background and the fact that we've seen Riku with bat wings in DDD. I can see him sprouting wings, claws and maybe a tail and gliding, or floating around, clawing enemies in a similar manner to the way Sora fights in his anti-form in KH2. As for Kairi, she could turn into, I don't know, maybe a unicorn? That would be downright adorable and it would kind of suit her, considering that she's a P.O.S. and that unicorns are pure and good and filled with light, just like her. When she transforms, her key-blade would become her horn and she'd be able to run pretty fast, have heightened magic and have some unique charging and slashing attacks. I don't know, these are just some ideas I came up with.
I'd also really like to see a Lilo & Stitch Hawaiian setting. They already used part of the original movie in BBS, so this setting would likely take place somewhere after that; probably when Lilo and Stitch are busy rounding up Jumba's other 625 experiments. And Gantu and Dr. Hamsterviel would have to make an appearance at some point.
Other Disney movies I'd really love to see featured in KH3 and/or future KH titles include Treasure Planet, Atlantis: The Lost Empire, Robinhood, The Sword In The Stone, The Black Cauldron, The Jungle Book, Sky High, The Chronicles of Narnia: The Lion The Witch and The Wardrobe, Frozen, Wreck It Ralph and Toy Story and/or other Pixar movies. I think it's very strange that we have yet to see any Pixar films featured in Kingdom Hearts. I understand that they were initially going to include Toy Story in KH2, or at least include Woody and Buzz as summons. I wonder why they later changed their minds?
I'd really like to see all 3 major hub worlds (Traverse Town, Twilight Town and Radiant Garden) featured in KH3 and for them to be bigger and better than ever, with a culmination of all the areas and shops from the previous games. I also think that if they do include Traverse Town, they should use the updated music track from DDD.
Personally, I wouldn't mind seeing some Star Wars and/or Marvel stuff in KH3. At the very least, I think there should be a Captain America shield for Goofy, Loki's scepter for Donald and a Star Wars inspired key-blade for Sora. It would have a glowing blue, or green blade, reminiscent of a lightsaber and the end would be shaped like the Rebels symbol. Riku could have a red one, with the Imperial symbol.
Lastly, I have one more idea that I think would be cool and that is…Wait for it…Gummy Mecha! I love building and flying gummy ships in KH2 and I think they should take it to the next level in KH3 and allow us to build mecha out of gummy blocks. How exactly would this translate into the gameplay? I'm not sure. Maybe gummy ships could transform into gummy mecha and vise versa and maybe during some gummy flights, you'd land on some sort of asteroid, transform into mecha mode and battle some powerful boss enemy? I don't know.
I realize some of these ideas are a little out there and I'm sorry that this post is so long.