Time of pointless speculation!
Melee had 2 Mother stages (Onett and Fourside) and new remix songs
Brawl had a new mother stage (New Pork City) and also new remix songs
Sm4sh had also a new mother stage (Magicant) and also new remix songs.
But, here's the weird thing, Ultimate have no new stage and no new remix songs... which is weird, I can understand that a stage would take more time, but no new Mother music?
Then you think about what Sakurai said, one character + stage + music, and it kinda fits... the stage and music that is missing is only missing because they are making a character for that DLC.
So, if they are working on a new Earthbound character, let's see what characters hasn't been show as spirits and could make it, and why they also could NOT make it (I know spirits doesn't deconfirm anything but let's pretend it does because talking about every single earthbound character would be too much work).
Let's start with Ninten:
+ He hasn't been show as a spirit (which is pretty good), he could work and people has been asking for him and we finally would have the three good bois in smash all together, he could come with any stage from mother 1.
- Sakurai said that the DLC characters would be new characters, Ninten would share some movements with Ness and Lucas (like a bat attack), which makes him kinda echo, so unless Nintendo found a way to make him 100% original I don't think he will make it (but Nintendo can surprise us!).
Masked Man:
+ As long as I know, he also hasn't been show as a spirit, his moveset is more easy to make original than Ninten.
- Samus lost her alt of dark samus and then dark samus got into smash, meanwhile Lucas still have his Masked Man skin, could be a hint that Masked man may not make it... also, not sure what stage could represent him.
Porky:
+ We know why he could make it, we talked about it over and over, he is a great main villain, could come with a creepy stage like Devil's machine or something similar and with his boss theme from earthbound.
- It is very probable that he is gonna be a boss, this doesn't disconfirm him but this could be the reason why he is not gonna be playable... but also being a boss will make him more easy to make as DLC, since they would already have the models and a final smash.