Like most people here have already said, stages and alt costumes. I would gladly take DLC characters too, but the roster is already huge and really good. Plus, I'm sure in a few years development on the next Smash game will start, and it would make more sense for them to develop new characters for that game rather than adding a new character to the old games and new game at the same time. Sure I would love to see Ridley, Isaac, Mewtwo, Ice Climbers, Lucas, Wolf, and a Zelda newcomer added as DLC, but I don't expect it to happen and won't be upset if it doesn't
DLC costumes and especially stages are really likely imo, I'd love for Great Bay, Fountain of Dreams, Fourside, Onett, Temple, Luigi's Mansion, Big Blue, Pirate Ship, Frigate Orpheon, Bridge of Eldin, Delfino Plaza, Brinstar Depths, Kongo Jungle, Skyworld, and Poke Floats to be re-added as DLC, and maybe one or two brand new stages too