This might sound far-fetched, but what about Yugi/some other Yu-Gi-Oh character?
Don't get me wrong, I'm not asking for him(after Joker, I'm probably never asking Sakurai for anything ever again, haha), but I feel like there's a surprisingly notable chance.
-I believe that, when Joker was revealed, Reggie said that it was "a taste of how they're handling the DLC characters". I wonder if this means characters who you'd think "oh, that could be cool, but it would never happen"?
-A Yu-Gi-Oh character would represent card games as a whole, something which, unless I'm mistaken, hasn't been represented at all before in Smash.
-Although he isn't from a video game, there have been several Yu-Gi-Oh games on Nintendo consoles(There were a ton on the Gameboy, for example, and two on the 3DS, though I think one was exclusive to Japan).
-Yugi is actually playable in a different fighting game, Jump Force.
-A card game based moveset could be quite interesting and unique.
The biggest things that would stand against this, in my opinion, are:
-Even if they are going for unexpected characters, a Yu-Gi-Oh character would probably be even more unlikely than Joker, especially since they didn't come from a video game, and most if not all Yu-Gi-Oh characters specifically came from anime, even if they have been in several video games.
-With Cloud, Shulk, and now Joker, there are already three anime-style characters in Smash, plus even more if you count Fire Emblem characters. I feel like adding any more could meet some backlash.
-Though I listed it as something working for his chances, the fact that he's in Jump Force could also make it less likely for him to be in Smash.
Sorry for the wall of text. Perhaps I'm reaching a bit, but this is just a bit of speculation. What do you think?
P.S: My apologies if this comment breaks any rules, I haven't used this website much.