For a Trailer theme, I‘d actually love to hear a remix of the title screen from Majora‘s Mask.
I always loved the shift from the happy melodie of clock town to the dark leitmotif of the game; something that reflects the character rather well.
By it‘s core, the skull kid is a trickster, a child, driven to evil by feelings of neglect and spite.
I feel to make Skull Kid like Skull Kid, Sakurai has to find a way to design a trickster type moveset, with maybe the occasional cruel attack.
Tilts, jabs and aerials can all be dance-like and quick, reflecting the more playful side of the guy...however, the Smash attacks are a different story...perhaps here the Mask takes control, dragging skull kid along like a lifeless puppet and ramming into the fighters...
Either way he‘ll turn out, I feel he deserves a spot on the roster. MM is one of the most recognizable Zelda games and one of the most beloved. The Mask itself has such an iconic design and Skull Kid is one of those rare reoccurring characters (OOT,MM,TP).
So yeah, Go Skull Kid!