Original arcade Donkey Kong to finally make use of the 75m stage.
The real answer btw is Pokémon. So many of the huge ones would make cool ones, as proven by Rayquaza. Articuno, Zapdos, and Moltres as a gauntlet or even better, reusing the multiple HP bar mechanic from Brawl's Petey Piranha fight. Giratina would make for a killer boss stage for sure.
I also think they should try some fighter-sized bosses at some point, where they're smaller and move faster but do less damage per hit to compensate. My two main candidates for that would be the Black Knight from Fire Emblem and the Boss from Metal Gear Solid 3 (the latter's stage would also be so cool).
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