Here are my predictions for the new game (it WILL be a new game):
Cut characters: Lucina, Robin, Roy, and possibly Corrin because the Fire Emblem series has gotten so much bigger since their release (we'll still have 7+ FE characters though). Wii Fit Trainer, Dark Pit, and Palutena, Mii Fighters, and potentially Rosaluma because they just aren't relevant anymore, and Cloud / Bayo / Ryu will not return because they're 3rd party DLC. Finally, at least two Pokemon will be cut and/or replaced with more current ones, most likely Lucario, Greninja or both.
Moveset changes: Mario's moveset will be revamped to reflect his appearance in Odyssey, Link's replaced to reflect BotW, Luigi's to reflect the Luigi's mansion series, Ganondorf's to reflect Hyrule Warriors, Zelda's to reflect either BotW or Hyrule Warriors, and Samus's to reflect either Prime 4 or Samus Returns. Wario, Bowser, Bowser Jr., and possibly Sonic will also have revamped movesets.
I'm not too keen on predicting new characters since we know their choices can be so random, but I'm counting on a new Kirby Character and Rex/Pyra sound possible.