Jigglypuff should receive minimal changes... aside from ignoring the fact Jigglypuff was designed by apparently an inept group of human beings in Brawl.
Just give Jigglypuff a similar playstyle and hitboxes to Jigglypuff's appearance in Smash 64 and Melee, although I'd like to see Rest have more visible hitboxes (like all the attacks we've seen so far), and perhaps wider range.
Also, I'd make Jigglypuff A LOTTTTTT faster on the ground. Jigglypuff's ground speed in Melee had a lot to be desired, but Jigglypuff's speed in Brawl (the worst in the game) was pathetic game design.
(Reasons #4,586 and #9,234 that I am happy Game Arts won't be the primary developer of a Smash Bros. game ever again)
They need to fix F.L.U.D.D.
Preferably they'd remove that move, but it seems we're stuck with it.
They need to actually give it tons of knockback so it can actually serve as the original purpose in Brawl... as a kill move via knockback.
That or just change it to a move that is meant for doing damage slowly but surely (kinda like a combination of Charizard's flamethrower and Fox's laser).