Warning: this is a move suggestion for King K. Rool based on something he doesn't personally use in Donkey Kong games, which may be seen as a crime by disciples of the "MUH MOVESET POTENTIAL" philosophy based on the fact that what defines "uniqueness" is the number of moves they personally use in their source games and the ability to port them directly to Smash without the need of creative liberties.
Joke aside, what do you think about a
Klobber as a down special for K. Rool? What should define K. Rool's playstyle should at least be stage control, and I'm sure this would be a great opportunity to contribute to that. Here's how I think it could work: K. Rool summons a barrel in front on him, if there's a grounded enemy not far in front of K. Rool, the Klobber will immediately reveal his identity and run, otherwise it waits until there's an enemy close to him. Anyone who is hit by a running Klobber will receive some knockback and damage. Whenever the Klobber hits an obstacle (such as a wall, or an enemy, including one who's using a shield), he will stop in his race and will know a cool-down period of a few seconds before he can start running again. If he's waiting inside his barrel, he can be picked up as an heavy item, similar to the regular barrel, and thrown, though as a thrown item the Klobber will be less powerful than a regular barrel. If K. Rool summons a Klobber, he can't summon another until the first one disappears.
The Klobber is green most of the time, but can also be yellow or grey at random. This, however, is purely cosmetical and has no effect on the gameplay (contrary to the source material where yellow and especially grey Klobbers are particularly dangerous).
The K. Rool moveset I have in my mind has as special moves: blunderbuss stuff (neutral B), krown toss (side B), helipack (up B) and a toned-down kannonball barrage (down B), but I've thought about trading the later for the Klobber. Not only it would be a way to put emphasis in the stage control aspect of K. Rool's playstyle, but it would also make him feel like a representative of the Kremling race, and not just of what he does in his boss fights. The (toned-down) kannonball barrage, on the other hand, could potentially be his up smash instead, where he shakes the ground and makes two kannonballs fall from above next to him on the right and left.
The butt-bounce I think would work as a down smash, as an added flavor it would only make the enemies fly away if they're hit by the shockwave around K. Rool, otherwise if they're hit by K. Rool himself they're burried. But it seems meh as a down aerial, and it would be terrible if is was a down special.
I also agree about the belly flop being a much better idea for a down aerial. Part of how I envision K. Rool as a fighter is a sort of "hit and run" gameplay, someone who overall dislikes melee combat asides from charging. As a way to put emphasis on that, I think the belly flop would work as a reverse sex kick (a property only Dr. Mario's neutral air has in the entire game), a move with a single hitbox which lasts for long, but contrary to regular sex kicks become stronger in the move's later frames. At the beggining of the move it will inflict weak horizontal knockback, but close to the end of the move it will become much stronger and have spiking properties. Combined with K. Rool's high salling speed it means that you must start the move when you're well above them and make sure you fall in order to hit them during the hotbox's spiking properties duration.