Ok here is my prediction after thinking about it. Imagine a world where these trophies fight for a living, sort of like the beginning of Smash 64 when the dolls were setup to battle. In the screens we see Mario going against Kirby, and Mario wins, so Kirby transforms back to a trophy, yet he comes back as him and Mario ackknowledge that they had a good fight. However perhaps one or some of the trophies (including meta knight, even though he is normally seen as a vigilante instead of an enemy in the kirby games) because fed up with the endless life of fighting, and decided to revolt, whether it be not transforming into a trophy while dying, or just acting all stuck up and not conforming like everyone else, so we see the halberd as meta is about to take action. because this happens, I think the world is corrupt and the characters, such as Mario change into these monsters, sort of like the Heartless from Kingdom Hearts. Then the story mode could be the character you chose fighting through matches of enemies to help save your fellow trophies from this darkness.