1) Rex/Pyra
I feel like Xenoblade will definitely get a 2nd rep and it will be Rex (unless there's another potential character from XC2). Don't know much about Xenoblade but I love their designs.
2) Bandana Dee
My personal most wanted newcomer. He's been recurring in pretty much every Kirby game since Return to Dreamland. He's been playable in 3 of those games, including Star Allies. Having a plot central role in Battle Royale. Also a spear would be a unique addition to the roster.
3) Gen 7/8 Pokemon
I think it's highly likely we're getting a Pokemon whether it'd be Gen 7 or the unannounced Gen 8. My guesses would be either Decidueye, Mimikyu, or Zeraora. Decidueye and Mimikyu both the most popular introduced within Gen 7. Zeraora has not been released yet but it feels like he's designed to be a fighter and would be very marketable.
4)Spring Man/ Ribbon Girl
The two most well-known faces of the ARMS series. Has a decent chance due to it's relevancy/recency and how much promotion their series has gotten.
5)Rhythm Heaven Character
A popular series and they would fit into an "out of the blue" character just like Wii Fit Trainer. My guess would be the Chorus Kids.
6)Takamaru
Smash typically has a "retro" representative for each iteration. Melee having ICs, Brawl having G&W, Sm4sh having Duck Hunt. I feel like this time around, it would be Takamaru.
7)Dixie/K. Rool
Donkey Kong can definitely use another character and either of these two would be most likely for their series. K. Rool being an extremely popular choice since the ballot days.
8)Toad/Captain Toad
Captain Toad has been gaining a lot of prominence especially with his game coming to the Switch. I wouldn't be surprised if he was added or another version of Toad.
9) Fire Emblem Character
Don't know much about Fire Emblem, other than the fact that it's very popular nowadays and has a HUGE pool of characters to choose from (hence why it had 6 characters in Sm4sh). If this is a new game, I'd think an existing FE character would be removed in favor of another more popular or recent one.
10) Isaac
Golden Sun has not seen a game in quite a while so I put him on the lower end of the list, but he still has a very vocal support base that wants him in the game.