I don't want to take this too far off topic, but I feel like there are so many more potential candidates this time around compared to Sm4sh. If true, this means the proverbial competition in terms of potential characters is hotter than ever. In other words, I'm not entirely sure that bodes well for characters like Krystal (or heck, even other popular characters who just aren't marketable or relevant in some shame or form like K. Rool [and this comes from someone who hates the "relevancy" argument]).
Barring Inklings, Nintendo has many other new IPs, 3rd parties, and older IPs with new/refreshed content to draw from (not in any particular order):
1. ARMS
2. Xenoblade Chronicles 2 (like you mentioned -- HOPEFULLY not Rex but unfortunately most likely [other characters/blades more interesting IMO])
3. Fire Emblem (either Shadows of Valentia or new upcoming Switch game)
4. BotW (like you mentioned)
5. Mario Odyssey(?)
6. Rabbids (can't forget they LITERALLY made a crossover with Mario -- could be WTF pull)
7. Crash Bandicoot (as per the recent direct)
8. Cut Smash veterans
I'm likely forgetting many others. The Switch has been so on fire that it's difficult to keep track of everything involving it.
Heck, this isn't even considering the possibility of INDIE game characters. I believe it is now more than ever likely that we will receive indie representation in Smash. Nintendo has really been hammering in the support for indie games on the Switch. I mean, even Undertale made its way on there!
I feel like I addressed this already with the "viable" component to my argument, but I'll address these individually
1. ARMS, I doubt it lined up with the roster selection, and the game hasn't done all that well to make DLC, but if it does, it's one of the few on this list that seem realistic.
2. for XC2, it's going to be the protagonist or no-one, it's just the pecking order for character selection. pretty good chance though.
3. FE has too many characters, there's no reason to continue to advertise it either as it's quite successful, it also has Heroes making a nice advertisement setup for it, even sakurai said corrin was too much, unlikely that we're getting Celica/Alm
4. BotW is pretty legendary, and likely lines up with development, even though it's a one-off I could see characters from it.
5. who would be from odyssey? I can't think of anyone who you could even make a kit for, even pulling one out of their ass
6. I don't think people would appreciate any of the Rabbids over Rayman, who had the fake leak to sky rocket his popularity
7. He's a third party character, I don't know if he has the draw but over Banjo he's going to be appearing on the switch, depends on whether or not Sakurai was aware and if there was a high request for him in the ballot.
8. There's only two smash vets cut that anyone wants so that doesn't take up mcuh space.