Sounds like a music rhythm action game. Seems fitting with how important music is to Disney movies.
So you think it could be a rhythm game? I doubt that, surely Square would release a rhythm KH game through their Theatrhythm series, like they already did with Final Fantasy and Dragon Quest. I don't think that Melody being in the title implies that it is a musical game. There is little reason behind KH spinoff titles, I think they just pick things they think sound cool. 358/2 Days, Birth by Sleep, Chain of Memories, Dream Drop Distance. My guess is that it plays very much like most of the other spinoffs do, with similar action RPG gameplay to the numbered titles, set between KH3 and the upcoming KH4. Possibly Switch exclusive, or multiplat including Switch (Switch/PS4/PS4/XB1/XSX).