heres my run down of possible RTDL FS
Ultra Sword: it was featured most prominently in the game and makes the most sense for a FS. and i really wanted it for kirby (and i love how its a multi hit move rather than one large strike that covers a large portion of the stage). although, i think it would be awesome/humorous if the sword changed with every strike, like how in RTDL it can be a meat cleaver, a bo staff, a fish, a pirate scimitar, a sword, a fan, or the galaxia sword.
Monster Flame: would just be a clone of Marios Fs but can be aimed slightly
Flare Beam: would just be a clone of pikachu's Volt tackle
Snow Bowl: could have some fun with this one, turning into a giant snowball and rolling into people, trapping them and then exploding after a certain amount of time.
Grand Hammer: This one i think would be awesome. you can charge it up in the same way (albeit pressing buttons rather than shaking) as in RTDL with each level being more powerful than the last. the highest damage output could be directly underneath the hammer, while the shock waves can push people away. and just like some final smashes, if you do it off the edge, you could ko yourself.
then you have the generic crash kirby which would just be a clone of a smart bomb.
Mike kirby. this would also be a fun one nodding to kirby's terrible singing abilities. it could be one giant blast of sound waves, or a couple.