I'd like to see Sing combined with Pound. Firstly, make Sing useful at last (i.e. have it keep opponents asleep long enough for you to attack them). Secondly have pound do massive damage and knockback against a sleeping opponent. Of course this would be hard to pull off, but it'd be deadly if done right (much like Melee's Rest). This would be a reference to Jigglypuff's move from the games, Wake-Up Slap.
No massive overhauls to her attacks, please. I see people taking a chainsaw to her moveset. Sing, Rest, Rollout and Pound are Jigglypuff's iconic moves, not only in Smash but in the Pokemon games themselves and even the anime to an extent. Sing and Rest just need to be made useful. That said, Hyper Voice and a Fairy move would be nice, but I still prefer the above.