Anyway, about Jigglypuff. I think in order to make any kind of fix to Jiggs, we first have to talk about Rest. The basic problem with Jiggs is that Rest is such a preposterously strong option that Jigglypuff has to be terrible in every other way or it would completely dominate the game. We can't give Jiggs any legitimate combos that aren't wall of pain because they'd just lead into Rest for a guaranteed 0-death. She can't have good mobility options and Sing can't be buffed because then she could land Rest too easily. I think any talk of fixing Jiggs starts with the return of Brawl Rest. As well as not breaking the game quite so bad, it also guarantees a Star KO when it kills, which is nice because the opponent doesn't get a free punish when they come back. Note that if Jiggs has legit combos into Rest, she could still get Melee-like damage out of a situation where you could land raw Rest as a punish, by comboing into it instead.
Now that Rest is no longer a super gamebreaker we can talk about how to fix her other moves. Clearly her other B moves are junk. Pound is useful as a recovery move so it can stay, but Rollout and Sing are both garbage. I agree that I think making Rollout jump cancellable is a stupid option. I'm not even really in favour of storable Rollout, because I don't really see how it would fit in with her game. Where would you use it? I don't see it comboing in any situation and it still comes out fairly slow, so it'd probably only work as a whiff punish and she already has decent whiff punishes. Honestly, I think the move probably just needs to get the boot altogether. It's not iconic to Jiggs in any way, nor is it very useful. It should probably be replaced by a fixed Sing that either fires a projectile or can be cancelled with shield, and then she gets some kind of new Up B that both serves as a recovery and has some combo use after Uair.
As for normals, honestly I think Dair to Rest should go back to working properly like it did in Smash 64, with the last hit of her Dair having a weak meteor effect so you get the pop-up and Rest links easily. Yes, it's still technically possible without it, but it's so unreliable and the risk is so great it's almost never worth it to go for it, and characters like Lucas get equal if not greater reward off a Dair already. If Jigglypuff gets in close enough to hit you with all of the hits of Dair (keep in mind she has no good disjoints, most characters should be able to wall her out), she deserves to get some moderate reward for it. I'd also support some kind of buff to her pokes, probably a pop-up on F-Tilt similar to Yoshi so she could Nair or Fair afterwards.