Since other people are doing it, **** it here's my predictionish roster
Paper Mario: a popular and relevant character with the potential to be a unique fighter, if any mario character gets in it should be him imo.
Dixie Kong: Tropical Freeze's timing makes her a perfect candidate, she's popular too and would have an interesting moveset
K Rool: The ballot is likely to inform the new roster and K Rool is ridiculously popular and would be unique, may be less likely that some of the others but has a decent shot.
Impa: Zelda needs another rep imo and no one is important enough to the series as her, hyrule warriors probably caused her to do decently well on the ballot so it's not out of the realm of possibility at all imo.
Bandana Dee: Important, relevant, unique, the perfect combo.
Wolf: A no-brainer given the fact that Star Fox Zero exists.
Decidueye: Gamefreak is pushing him quite a bit, he was the only new fighter in pokken dx's base game after all. He would've been chosen before sun and moon was out so popularity wouldn't be a factor, instead his unique design and moveset potential would likely inform sakurai's decision, and he may want to opt for a grass starter to complete the trio.
Isabelle: A staple to animal crossing already and ridiculously popular, I'd be shocked if this didn't happen.
Elma: Xenoblade X's timing makes her a prime candidate and her moveset potential is massive.
Icies: duh...
Takamaru: He's been considered twice but was kept out for being practically unknown to the west, since Smash 4's roster was planned his game was ported over here and his popularity among the smash fanbase has grown exponentially.
Rayman: May not be the most likely choice but he was a trophy in the last game, the only 3rd party series not playable to get one, he likely performed well on the ballot and Ubisoft and nintendo have had a great relationship as of late, I could see them asking for his inclusion like kojima did with snake.
and for fun I made a less realistic one too