I'm late to this thread, but honestly, I'd like to see as much music as Shimomura could possibly fit into this game! But as for what I absolutely want the most in there, I just want more combat music, character themes, and boss music than anything else. I know there will be a ton of Disney world music and combat music from the Disney worlds, but I hope there will be a bigger focus on original world that anything else, but as long as Hollow Bastion, Twilight Town, and Scala ad Caelum music are in there, I would be a happy camper. Xion's theme and Namine's theme are a must, especially since the trailer showed how slower, and more emotional character themes can work, with Roxas' theme, the Other Promise.
Boss themes are what I really want though. You could literally name off every boss in the series and I would want their theme in the game. Darkness of the Unknown and Rage Awakened are already confirmed, and both of those levels look hype as heck, so off the top of my head, some boss themes that I want turned into rhythm-based boss battles are Forze Del Male (Riku-Ansem and Ansem SoD), Lord of the Castle (Marluxia), the 13th Struggle (Organization XIII, in general), Vim and Vigor (plays against most Disney bosses in KH2), Unstoppable (plays against various Unversed fights in BBS, and Lump of Horror boss in KH3), L'Impeto Oscuro (plays against Dark Impetus in BBS and Young Xehanort in DDD), and Oscurita di Xehanort (plays against Ansem, Xemnas, and Young Xehanort in KH3). Also, I don't know the names of the themes, but Anti-Aqua and Final Xehanort's boss themes in KH3 would be cool too. Above all, I want Vector to the Heavens (Xion) and Enter the Darkness (Vanitas). They could literally not have any of the boss themes I mentioned before hand, but if they had Vector to the Heavens and Enter the Darkness, I would be fine with that! Also, if it is possible to include every boss remix from the data battles in KH3: Remind, I really hope they do it!
And on a side note, I really hope they include the opening themes from the games that were sung by Utada Hikaru. Idk what they would have to do to be able to include them, but it would kinda weird to have a KH rhythm game without Simple and Clean, Sanctuary,etc. Just my (surprisingly long) thoughts on the subject!