I think the hard knockdown would probably need to remain tied to the Heavy version of Skyfall, which I would like to see kept the same as it currently is. I think the new Skyfall should be as follows:
Light - quick but short forward air dash, with a burst of crows propelling Thunder forward. This is purely movement/mobility related, and does not result in an attack. Maybe projectile invulnerable, but probably a chunky recovery time. If it doesn’t have the invulnerability, I don’t think a long recovery is needed.
Medium - a down-forward diagonal attack, allowing Thunder to come at the opponent from a potentially unexpected direction. An overhead/high attack which opens a combo if the opponent is standing/crouching, but otherwise causes a soft knockdown.
Heavy - As it currently is; an overhead attack, as Thunder slams straight down to the ground from the air, knocking the opponent down.