I want to see if Jigglypuff gets a new Up-B. No one used that move competitively, as far as I could see.
EDIT: greninja'd while using greninja
Nonono, Sing needs to stay. It's obviously a bad/mediocre move, but putting enemies to sleep via song is Jigglypuff's biggest motif, it would be like removing Mario's fireball, maybe even worse. It really does need to get upgraded, however, in some way or another. If anything, I think Pound should be replaced. While it's a surprisingly useful move for what it is, that doesn't make it any less generic. I think Jigglypuff might appreciate a projectile more anyway, assuming that Disarming Voice or Dazzling Gleam worked as (good) projectiles.
Sing can punish spot dodges and rolls behind without needin' to react. If you can't lessen Sing's sleep duration by control stick rotation/button mashing, it becomes a free Rest. Furthermore, makin' Rest's hitbox not having any pauses means it can't be dodged by spamming spot dodges, giving Sing more consistent use. Rest isn't broken in vBrawl and probably won't be in Smash 4 either, so that'd be a great buff to Sing without bein' broken. It'd also make Rest moar useful. Jiggs should have some "reliable" KO power since she's so bloody light. Doesn't have anythin' else to her name since great recovery only goes so far when ya pop like a balloon.
But Rollout is so fun to use, don't forget that us casuals play this game too. I think that Pound could easily be replaced with Disarming Voice, which like you said could be a blast projectile similar to Samus' normal B, only with some differing qualities.
I've found Rollout really useful but I mostly play against fellow casuals, so that doesn't necessarily mean a whole lot. But I never found it easy to kill yourself with, it's easy to turn around. It's certainly possible to kill yourself with it, but you'd have to make a pretty major derp.
Rollout is still one of my favorite parts of playing Jigglypuff, it's certainly an entertaining move to use, even if it's (apparently) not very good in competitive matches. Pound is relatively boring despite its higher usefulness, it's literally just her punching in front of her.
Alternatively, Disarming Voice could use the same b-tilt as Sing. Maybe pressing a certain button alongside up-b could make her using a damaging sound wave attack instead, with a similar animation/radius to sing.
They nerfed Rest and Rollout and Sing have never been good.
If they buff Sing then Rest would have to be nerfed
If they buff Rest then Sing is useless
Rollout has always been bad