Yeah well there's always the possibility that skull kid and the moon are both assist trophies in ultimate, but that seems like a stupid decision, and there's no way skull kid would be removed entirely, he's just too popular, so really I can't say I can see how he isn't playable, but then again I feel like I'm jinxing it by saying all this, it's giving me flashbacks to all the ridley speculation in smash 4
Another thing about assist trophies. Sakurai mentioned there were "over 50" of them. So far, 40 have been confirmed. Here's how each franchise is represented so far (excluding Pokemon):
Three representatives: Legend of Zelda
Two representatives: Super Mario Bros., Metroid, Kirby, Star Fox, Earthbound
One representative: Donkey Kong, F-Zero, Fire Emblem, WarioWare, Pikmin, Animal Crossing, Xenoblade, Splatoon, Sonic the Hedgehog, Metal Gear, PAC-MAN, Megaman, Bayonetta, Castlevania
No representation: Yoshi, Ice Climber, Game & Watch, Kid Icarus, R.O.B., Wii Fit, Punchout, Mii, Duck Hunt, Street Fighter, Final Fantasy
Thirteen are represented by (as of now) franchises without a featured playable character.
Given this info, Legend of Zelda already has more reps than anyone else, including even Mario Bros. We have probably fifteen reps yet to be confirmed at most. As of now, Legend of Zelda has no representation for that behemoth among Zelda games that is Breath of the Wild. Which would leave us with - being generous - fourteen other assist trophies to be spread out among other worthy franchises like Donkey Kong, Animal Crossing, Kid Icarus, or Final Fantasy, who have next to nothing as of yet. With no Breath of the Wild reps and not even Tingle revealed as of yet, I can't see Skull Kid getting picked for assist this time around. The main problem I see confronting him is whether or not he'd be a boss. And considering his small stature, and the fact that he meets all of Sakurai's qualifications for making a Smash Bros. character quite easily - not to mention his booming and enduring popularity among fans and Nintendo developers alike - he seems really, really likely. I mean, the dude received his own unique Melee stage, for god's sake. His face: an actual stage. That’s how iconic he is.
Alternatively, Skull Kid may not be in the game at all, and Majora's Mask becomes a boss. I kinda doubt this though, as a move like this seems like it would please literally no one, and would likely only result if Sakurai can't think of a good way for Skull Kid to work as a character. Which, given the number of awesome amateur movesets we've seen on this thread alone, doesn't seem to be much a problem either. Not to mention, Majora's Wrath/Incarnation is a whole lot less recognizable than Skull Kid/Majora's Mask itself, and there are many more bosses worthier of inclusion in a boss mode (energy ball tennis game with Phantom Ganon, anyone?).
I'd bet money on Skull Kid's inclusion.