Personally, I think that at the very least, every franchise that has at least one playable character should have at least one stage. I am not saying if a series got a stage, that series needs a character (like some people use as a reason to support an Animal Crossing character, what about Pictochat and Electroplankton, huh?)... I am saying if a series got a playable character, that series needs a character (why this didn't apply to R.O.B., I don't understand but he could have gotten a stage from Gyromite
).
I would rather they add 50 brand-new stages and not bring back any past stages from SSB/Melee/Brawl. How many times do I have to see Hyrule Castle, Big Blue, etc.? If I wanted to play on these stages, I'd play Melee or Brawl.