Dixie Kong & King K. Rool
With the release of Returns, Donkey Kong Country seems to be seeing a resurgence, and Nintendo will want to capitalise on that. Not to mention, King K. Rool has a rather big push for him, as we all know.
Decidueye or Incineroar
We know that Sakurai reserved a slot for Greninja, and it's possible Sakurai did it again. I can see either Decidueye as a Ranged Fighter, or Incineroar as a Grappler-type Fighter. Ultimately, it would be "who does Sakurai see more potential/uniqueness in?"
The reason why I say either-or is because, as with Corrin last game, Sakurai has reservations about over-representing a single series. While Pokémon is a pretty big series, Sakurai literally just brought back Ivysaur and Squirtle, but he'd probably want at least one new Pokémon. Unless Game Freak's Gen I push is considered enough marketing this time.
Celica
Having just talked about Sakurai not wanting to bloat a single series too much, let's talk about Fire Emblem! Fire Emblem Echoes likely went into production around the same time as Smash Ultimate's plan was being made, so it would be fresh in Sakurai's mind. We know already that he likes Fire Emblem, and between Alm and Celica, the Priestess is less generic (sorry Alm fans
)
Not to mention the fact that Alm and Celica came pre-made (Sakurai would only have to wait for their redesigns to come in, which would happen pretty quick), so... yeah... there's that.
Ashley
I have already spoken about Ashley, she's popular in Japan, she has moveset potential, she'd be a unique fighter. So it seems like a given?
Isaac
This is just wishful thinking on my part, to be frank. Isaac has so much to give, Sakurai! Camelot too! Whyyyyyyyyy~?
Tails and Shadow
If any third party franchise is 'deserving' of multiple new fighters, it's Sonic. Sonic, Tails and Shadow are the three most popular characters in an massively popular franchise. There's a lot of sense in having them both.
Simon Belmont
Vergeben strikes again! Okay, but seriously; Castlevania is a popular series associated with the NES; and Simon has potential as a fighter.
Sephiroth
A character who would blow people's minds just as much as Cloud himself, and I'm sure Nintendo would want lightning to strike twice. I imagine he would be the last character reveal just before the game is released, like Duck Hunt's reveal last game, only on steroids.
Bandana Dee or Elma
Now... this depends on whether Sakurai really has an unbreakable bias in favour of Classic Kirby. This is the character I'd drop for a +11. Elma was unproven at the time of Smash Ultimate's beginning, so that's a factor against her.
Ken
As far as Echoes go, you really can't go wrong.