I think my only remaining 100% prediction now is that Skull Kid will not be playable. It's just too farfetched to be within the realm of possibility.
There are plenty of other arguments aside from "chairs" that people are making for Skull Kid.
- Missing his AT currently (the same argument was made for Dark Samus and is being made for Shadow and Isabelle)
- His MM presence has so far been "replaced" by the Moon in terms of AT representation
- A new item now (Ramblin' Evil Mushroom) functions exactly as his AT did but you fire it yourself (no longer an AOE)
- At least some MM music has been remixed
- Termina has returned as a stage (doesn't mean much but it theoretically could be his stage and YL's)
- YL's final smash is hidden (similar to Robin's not being shown yet when people were speculating Chrom), and many speculate it is Majora's Mask related.
- We do not know all of YL's costumes yet. It is very possible he has his MM items as an alt costume for this reason.
- Finally, Zelda hasn't gotten a rep since Brawl and its was just a semi-clone. The series needs a new rep, and Skull Kid is the next most logical pick due to being the most iconic behind Link, Zelda, and Ganondorf.