After Sephiroth, my gut feeling went to Reimu, Isaac, and Lloyd. It seems like each character breaks a pattern; of some sort (Lack of Nintendo connection for Joker, smaller third-parties for Terry, Spirits for Min Min, villains and already-represented, etc.) The ones that people seem to be the most unsure about are indies and Assist Trophies, which is what led to Reimu and Isaac (Though admittedly Shovel Knight could cover both grounds). Lloyd comes from the doubt surrounding the absent Smash 4 Miis.
Is this a stable theory? No. Does it have a solid foundation? No, it doesn't. Am I going to be taking this theory seriously? Heck no. It's mostly a hunch, really.