Eh, a non-playable Kirby Boss is inevitable - either Kracko, Heavy Lobster, or Dyna Blade, I'm betting, if not all three. Bowser, Rayquaza, and Wario are inevitably some sort of story-based fight in the game, as well as the Ancient Minister. Likely, Master Hand and Crazy Hand will return. Ridley is inevitable as a boss, playable or not, Mother Brain and Kraid also probably appearing at some point. There'll also probably some Petey-like bosses from other game series, ie minor reocurring characters that were bosses at some points, thus making things like Kamek, King Dodongo, Giant Bulborb, Eggplant Wizard, Yellow Demon(if Mega Man is in) etc... pretty likely. Also, of course final bosses, esp. Porky, Ganon, Ashnard, Andross, Eggman, Metal Gear REX. I could also imagine weird bosses from other series (If Isaac gets in, I want to have an epic battle against the wise one) like a Boss fight against Jimmy T. on Wario's stage, or destroying a damageable Landmaster tank.