Paper Mario:
The fans may hate the series by this point but it's still going strong lol, Paper Mario would allow them to represent a different aspect of the mario series while creating a unique character in the process, doing things like using his hammer and partners like current mario doesn't do. He's another character that just makes sense in my mind and feels next in line to be added of the Mario characters, plus...he can jump
Dixie Kong:
One of the most obvious choices, a relevant character who is also popular with ample amounts of moveset potential to boot, I'll be shocked if she isn't in
Impa:
One of the less likely choices but I feel Zelda is very much due a new character and with impa gaining notoriety thanks to Skyward Sword and especially Hyrule Warriors where she was actually the second character announced after Link I think she makes sense, especially since she's one of few reoccuring figures throughout the series.
Bandana Dee:
An obvious iclusion tbh, he's essentially become the fourth main kirby character, appearing as a playable character in RTDL and Rainbow Curse, as well as appearing in Triple Deluxe (where meta knight didn't appear at all), Planet Robobot, and Star Allies as well as a bunch of merchandise. It's clear Hal is pushing them and I wouldn't be surprised if he were to appear.
Decidueye:
Feels like the obvious pick for a Pokemon representative due to how easily they translate to a fighting game, as shown with Pokken where it was the sole newcomer to advertise the Switch port, the archer archetype could certainly lead to a unique fighter. Also while not a major factor it would complete the starter trio and while some may dismiss that as not mattering it could still be a factor. This spot could honestly go to any Gen 7 Poke but I feel Decidueye lends himself easiest to smash, pokes like Lycanroc and Mimikyu may have popularity advantage but the roster was decided before the game even came out and they have their own problems too with Lycanroc being bipedal and Mimikyu's body type not lending itself as easily to fighting.
Isabelle:
Isabelle's become a mainstay to Nintendo as a whole in recent years, Isabelle was right there front and centre on the amiibo festival box, every promotional image of the AC amiibo had her front and centre and she was even playable in Mario Kart alongside Villager. While she lacks obvious moveset potential, I'm sure if anyone can come up with a unique one it'd be Sakurai and Nintendo would probably want her in regardless since she's so popular with casual fans and gamers alike.
Elma:
The Elma vs Rex debate has been a very polarizing one and I've gone back and forth on it myself, I stand by the fact that Elma is the more likely candidate due to timing, as well as how clearly and easily she would translate to smash while being a unique fighter. Another benefit to Elma is that she represents a different part of the Xenoblade series that is likely to be revisited if the games ending is anything to go by and Monolith themselves they want a switch port as well, as such Elma probably has a future with the series which I think helps her case.
Takamaru:
A character that has been considered for Smash multiple times over and has yet to make it in due to his lack of appearances in the west, however his game released in the west in 2014 and being an assist trophy and mii fighter costume has allowed him to gain further exposure, now certainly feels like a good time for his inclusion, not only that but he would likely be a very unique character as no other fighter uses a fighting style like him.
Chorus Kids:
A Rhythm Heaven character was seemingly planned for Smash 4 and didn't make it for one reason or another (Priority for other characters? Technical issues?) regardless Rhythm Heaven is still a series that Nintendo is pushing, getting a release in 2016 after all which certainly helps them out, now seems like a good time to add them.
Wonder Red:
One of the more out there choices but I still think it's a definite possibility, for one, he has ample moveset potential which I think is his strongest argument, two, Sakurai is friends with the games director Hideki Kamiya I believe, while Bayonetta was added too, that was because of the ballot, not his own personal choice. The Wonderful 101, while not a commercially successful game was generally well liked and Platinum want to port the series over to the switch, and Nintendo have consistently been supporting Platinum with the Bayonetta series. I think it could happen tbh.
Rayman:
A character that just makes sense, Ubisoft and Nintendo have been very friendly as of late, Nintendo even allowing them to cross ****ing Mario over with their rabbids series and them publishing Rayman Legends in Japan. Rayman, as well as Globox and Barbara were included as trophies in Smash 4, the only non-playable 3rd party series to get trophies. And Rayman is generally a fairly big, iconic and recognizable character, there's honestly not that many left that would work in smash.