Kingdom Hearts: Melody of Memories features 31 Disney-based locations and 16 original Kingdom Hearts ones. There are 33 “boss” stages and over 140 songs altogether. It's a lot of content considering the type of game it is, and hopefully you enjoy what's in there, as more isn't coming.
Square-Enix has confirmed that there are no plans for DLC right now. Everything they wanted to pack into the game is there on day one. Not too often we see a full package release nowadays, so hopefully the game keeps players busy for quite some time!