I imagine Harmony Kirby being twice as powerful as StarRod Kirby & can do tons of damage to not only Nightmare himself, but to all of his demon beasts as well.
You know, I had a thought about if the Warp Star was infused with the power of the Elements of Harmony and Kirby inhaled it, He would be able to form the Rainbow Sword like from the Kirby Dream Land 2.
Implied vore.
that would be cool