So here is something interesting found in the Japanese Mario Maker stage trailer.
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Apparently, we only have room for one more stage unless another row is taken up. Another thing is that Sakurai said they can't make many more stages back at E3. So here's what I'm thinking:
1. The last stage wouldn't be a returning one. No matter how loved it was, it wouldn't be as cool as a new stage with new possibilities.
2. It needs to be from a game thats fresh, well known, and overall loved. They want the last stage to probably go out with a bang. So you need something amazing. I don't see a DKC, Golden Sun stage regardless of whether or not K.Rool or Issac get in.
So with that, I see only two possibilities. Splatoon or Starfox Zero. Both games are new and have potentially visually stunning designs. Both series are also well loved.
Of course you know where I'm going with this in the character discussion thread. That means potentially either Inklings or Wolf. I said it before in this post, but they probably want to go out with a bang. So bundling it with a character makes sense.
I could be wrong about there being only one more stage left, but between it looking like one more spot for both English and Japanese Stage Selection Screens and Sakurai saying he can't make many more stages, it just makes sense to me.