I said this way back in this thread, but since I can't find my old post, I'll just restate my thoughts on a Zelda newcomer.
It isn't farfetched to think that people want a Zelda newcomer. Considering the fact that Sakurai is giving love to franchises that didn't receive newcomers in Smash 4 (Donkey Kong and Metroid), Zelda is one of those franchises that hasn't really received a lot of priority when it comes to newcomers.
Of course, the big question is who would be chosen?
Well, if Skull Kid were to be chosen, I don't think Sakurai would look at him in the same light as K. Rool and Ridley. Instead, Sakurai would look on the Ballot to see what franchises have received requests for a fighter and only then would he choose the most popular.
It isn't unheard of, as this happened before with Cloud. Even if he wasn't as requested as other popular choices, people voted in a lot of Final Fantasy characters and Sakurai evidently took notice. Out of all the FF characters voted in, Cloud was the most popular, so he was put in.
Tl;Dr: Skull Kid would be a Cloud-esque pick in that even if he himself didn't receive a ton of votes like K. Rool did, the Zelda series had a lot of requests and out of all them, Skull Kid was probably the most popular.