With Kingdom Hearts 2.5 ReMix they have the opportunity to integrate the Mirage Arena with the PlayStation 3's and Xbox 360's respective online platforms (but this is all wishful thinking, since High-Definiton "remasterings" are usually just quick money-grabs). Onto Kairi actually being useful for once, like I stated in another thread: she now has the opportunity to go from damsel in distress to dueling damsel. What could develop her character is her thought of realizing that it is time for her to stop getting rescued by her "knights in shining armor" and start becoming one of those knights, in other words, give Sora and Riku a break for them to focus on the bigger threat. Personally, I am indifferent as to whether or not the remaining members of the former Organization make an appearance in Kingdom Hearts III as complete individuals. For all we know, their Heartless were never defeated. The only purpose they may serve is of NPCs. Perhaps KHIII may feature a gambling-like spot like Final Fantasy XIII-2 did and the owner is Luxord and you get to meet him once you've played each of the mini games the place offers. KH3 will not fail to deliver a nice piece of eye candy with the immense power the PS4 and Square Enix's engine can offer. And I agree with other members: I, in my honest opinion, cannot see any Lucas Film nor Marvel characters in the universe of Kingdom Hearts.
On regards of weapon customization, the only customization we may get for the Keyblade will just be the Keychains. I am indifferent as to the Theater Room featuring the customizations we gave the characters in that particular moment and yes, there should be a Sound Test option, but releasing soundtracks is something SE can capitalize. A commercial could just be one of the many trailers they will release. Noctis in KH3 could be an interesting superboss, like Sora enters a time portal that reads "This will lead to a distant future, possibly 15 years from now, it could be your final fantasy, so best be careful". While I am having an easy time picturing Noctis in the universe of KH for a brief moment, I fail to see how Sora may fit in Noctis's universe. Sora is also I character I may fail to take seriously in a battle against him, he is not someone I would like to hit. Do not get me wrong, I can imagine a fight with him, but not ACTUALLY hitting him, haha. Last but not least, I believe KH3 will feature a satisfying story since it IS the last chapter of the Xehanort Saga and a very long one and I believe Nomura-san is a man that learns from his mistakes and would like to develop KH4 and onwards with a clean and planned slate.