1) That the worlds feel disconnected from the plot as a whole. It's something I felt in most of the previous games in the series. You pop into a new world only to completely forget the actual conflict that drives the story to stop and help rescue a Disney character, and then suddenly and randomly you are rewarded with a keyhole or something. Please, make the plotline build tension and give a sense of purpose to the Disney worlds instead of simply ignoring the story and sticking cutscenes in between the worlds.
2) That Sora's character doesn't develop enough. I want to see him respond more emotionally to different circumstances. I understand he will always be sort of childish, but I don't want him to be so optimistic to the point that his character feels shallow. I want to see him angry, hurt, or even something SoKai. Just give him a bit more depth.
3) That the ending isn't sad enough. I've actually seen other people afraid of the opposite, that we get a sad ending, but I love them personally. And with the climactic ending we're getting I want it to be emotionally impactful. I want to be staring at the credits rolling and thinking, "Whoa." And my reason for saying this is, the sad stuff sticks with us longer. I want the whole fandom to be comforting each other and theorizing on how whoever's favorite character might not really be gone or might make a return. I wanna feel the aftershock of this game for a long time. We've waited for it for so long, it shouldn't be so satisfying and filled with happy endings that we move on too quickly after it's over.
4) That Cloud, Aerith, and the Hollow Bastion crew don't show up. I want some new FF characters too, but don't ignore the ones that have been around for 16 years already. They're ingrained into the series as much as any other character now. (Plus I love seeing Aerith alive and well...)
5) That Riku dies. I'm completely expecting to lose some main characters in the process of this game, but Riku has had some great character development and I don't want him to go. There's not really a technical reason I want him to stay, I just... like him. And I like his new hair... (´ー`)