I had a theory a while ago.
I've wondered why some characters' FS haven't been showcased yet. Characters like Diddy, Robin, and Young Link.
Young Link is the strangest for me because they could of easily showcased some sort of Triforce Slash like Link used to have and Toon Link still has. But nothing has been shown.
Now, Majora's Mask 3D was released around the time Sakurai would of finished his roster document. And Skull Kid has not been seen yet as an AT in any demo, image, video, etc.
I theorize Young Link has Oni Link in some way as his FS, and Skull Kid could potentially make the roster as a fighter.
All this right now is just my own brain scrambling for news in this drought. But, I'm hoping with Ridley's inclusion we finally get more villains this time around, and honestly Skull Kid would make sense to pair up with Young Link coming back. We also got Great Bay coming back, so we know Majora's Mask is being a little more heavily represented in Ultimate.
But I think we'll need more news on fighters to see if any of it will come true, since we have no idea what direction Sakurai is going with new fighters.