Captain Toad: Honestly, I'm not sure how he'll work. He can't jump. But he's finally given a distinct identity to a long-desired character, so I feel Sakurai will find a way to make him work.
Ashley: My personal preference to a Wario newcomer, and a highly requested one at that. I think she could bring several unique ideas as a moveset and would be a healthy addition to the roster.
Mipha: Zelda is due for a newcomer, and Mipha is my personal choice. Breath of the Wild is an insanely popular game, and the champions are all well designed characters with plenty of reasons to be playable characters. That being said, I think Mipha is the best candidate out of the champions. She brings that spear moveset people have always wanted (though they typically wanted it for a new Fire Emblem character) and she is a long-standing and important race in the Legend of Zelda franchise.
Dixie: Dixie has seen a resurgence in the latest Donkey Kong titles, on top of being a main character in 2 of the original 3 Donkey Kong Country games. She has certainly built a strong resume.
K. Rool: MOAR VILLAINS! K. Rool is an obvious choice as part of both the Donkey Kong franchise, and as an option to add some extra villains to the roster. While the games he was present in are old, he's still the most prominent DK villain around.
Marx: Marx is really just my personal preference for a Kirby newcomer. He's a real villain who could bring unique ideas to the roster using his ball, which can have different Kirby elements added to it, as well as utilize several of his other moves as a boss. I think Bandana Dee is probably more likely, but I prefer Marx and think he at least has some chance, so he's on there instead.
Sylux: Basically I just find him more likely than Ridley based on Sakurai's comments towards Ridley. I would PREFER Ridley, but I think Sylux is more realistic until we actually see playable Ridley. Also it has been hinted at several times that Sylux is going to be a big deal soon.
Decidueye: I see Decidueye as the most likely Pokemon from gen 7 based on design and moveset options, but I think that space could be open to other options. Mimikyu or an Ultra Beast come to mind as alternative options to fill that hole. I do firmly believe we will be getting a playable gen 7 Pokemon, though.
Lyn: Lyn is a very clearly popular Fire Emblem character, even before Fire Emblem Heroes came out. She won the pre-release Choose Your Legends ballot by a convincing margin, and she's been an AT in the past 2 Smash games. There hasn't been a 100% new Fire Emblem main series game released recently, and we still know nothing about Fire Emblem Switch. I think her only competition as a Fire Emblem character would be Celica.
Takamaru: I see Takamaru as a realistic classic addition, especially knowing he was on the table for Smash 4, which the roster had been completely set before his game was localized outside of Japan. I think he gets his chance in Smash Switch.
Isabelle: Animal Crossing is an extremely popular game, even to people who don't really game otherwise. Isabelle has arguably become the face of the franchise, considering the actual "face" is an avatar character. She helps the player extensively in the most recent Animal Crossing titles, and has a lot of personality to help develop an interesting character. I do believe she'd be a semi-clone of Villager, sort of a Wolf-tier clone.
Chibi Robo: Let's face it, Chibi Robo as a franchise is in a rough spot right now. The first game was incredible, the second game was pretty good... Then the most recent two happened. However, Chibi Robo is such an interesting character! There's so many different things he's capable of, and if his franchise was able to stay the large-scale adventure style game the first one was, I have no doubt it'd still be great. I think, given a proper chance at gaining some popularity, Chibi Robo could take off. Smash has proven itself as an effective form of advertisement, and even though I wish it wasn't used this way, I think Chibi Robo is a good option to fit there. And also, I think Chibi Robo could turn into another one of those Captain Falcon characters who continues to exist only in Smash. It'd be a shame to not ever get a new Chibi Robo adventure game, but if he can live on in Smash, then so be it.
Rex & Pyra: I don't think they really need a ton of explanation. Popular game, growing franchise, can be distinct from any other character on the roster pretty easily.
No new third parties: I know many of you want new Third Party characters, but there just aren't any that stick out to me. None that really sit on the same tier of Pac Man, Ryu, Mega Man, and Sonic. Bayo is sort of a different case from them, as she's basically a Nintendo character anymore... I'm sure we will get a new third party or two, but I'm not going to be able to predict them.
ARMS: I don't think ARMS gets a character before DLC. I think ARMS might have come along too late. Don't reply to this, don't flame me about it, I'm going to ignore any post that tries to argue about ARMS with me. I've had that discussion enough, and I know where I stand on it. Save your ARMS soap box for another day.