I understand the sentiment towards Kafei because of his trite playability, but as far as moveset potential goes, he's basically a weaponless Young Link. Lending any tools would be a stretch from his in-series depiction, crossing the threshold of complete invention. I can't argue against this because Captain Falcon never fought outside of the Blue Falcon before Smash 64 (beyond his bounty hunter background, but that's irrelevant), but I do believe there are more iconic representatives of the Zelda franchise. Kafei was playable, sure, but so are the seagulls in The Wind Waker.
Skull Kid & Majora's Mask would bring unprecedented originality to the table. A dual-character, capable of switching between Skull Kid's grounded control and Majora's Mask's aerial control, all within the same moveset. On foot, Skull Kid runs around, but in the air, Majora's Mask drags Skull Kid like a puppet, taking a leaf from its floaty movement in the final battle. They would form a symbiotic relationship akin with Olimar and his Pikmin, with the tight groundplay of Sheik and the levitation of Mewtwo. With the added potential of separating, taking control of Skull Kid while leaving Majora's Mask—like a bumper—to hover in a fixed spot, surging darkness on those unlucky foes to make contact with it would bring something unseen to Smash. To forcefully separate a character in two, potentially controlling either half or even switching between them... Think about it. A character that could move as one unit, or as two—with one becoming immobile. Skull Kid would fall and lie limp like an abandoned marionette while the player controls the speedier, completely-aerial Majora's Mask. The ability to divide and conquer the stage like so only brings to mind Zelda's trapping with Din's Fire. This would be new.
I only foresee Kafei being an alternate Link, using items not yet represented between Link and Toon Link. Kafei would simply be a visual substitute for yet another actual Link.
Well, with the reporting, anyways.