Not a big fan of Onett in Smash, despite of my love for the Mother series. I don't know if a redesign should be considered or if it can stay as it is now.
Casually, it's not that bad but it quickly becomes tedious, notably because we know the stage since Melee and it's always the same approach between players. Can be fun but never a blast for me.
Competitively, it's currently not an option because of walls, low ceiling and obvious hazards and I don't think an overhaul could do something very interesting since you should take off all the things that make it unique.
But I do think Mother needs a competitive stage ; Onett is just not a good material for it. I already shared to Sandfall my ideal Mother's stage, which is based on the famous Battle HUD : psychedelic background with vertical moving platforms like in earthbound:
This could give freedom to designers in addition to provide a fun stage for competitive stuff. Currently, Onett is "OK!" without being stunning but I can't think of a good reason for changing it. But because of this thread I'm really eager to see full original stages from the PMDT in the future, not only minor changes.