The roster rounds to a nice 80 characters (84 counting Mii Fighters and Pokemon Trainer separately), which Nintendo can use when marketing the game. There are 17 newcomers, which, while the same number as Smash 4, is really only 12.5, as Dixie, Chrom, Isabelle, Dark Samus, and Daisy are based off of other characters, making their development easier. Characters like Incineroar and Bandana Dee can also borrow some moves from other characters.
Dixie Kong: A popular, relevant Donkey Kong character who was originally planned to be in Brawl. I imagine her being a semi-clone of Diddy Kong (or maybe an echo), which makes her easier to implement as a character.
Incineroar: I think he's the most likely Pokémon newcomer given that he could bring a unique fighting style to Smash, being that of a grappler/wrestler. That fighting style is classic, and I think Sakurai would want to represent that in Smash Bros. through Incineroar.
Erdrick: One of the leaks today I read really got to me when it said that the Japanese weren't as hyped about Smash 4 as Americans were with picks like Little Mac. I think that makes perfect sense. Characters like Little Mac and now Ridley cater primarily to American fans. Case in point: Little to no one played Ridley in the open tournament tonight compared to characters like Ike, Ness, and Inkling. I feel like this "Ultimate/Special" version of Smash Bros. is meant to cater to both western and eastern fans equally. Characters like Ridley and Geno are largely western picks, whereas characters like Ashley and Erdrick are largely eastern picks. Of course, there is overlap in terms of western/eastern appeal with all of these characters, but certain characters cater more towards certain demographics this time around. Plus, Sakurai might as well take all the Square Enix characters he can while he has them at the table, right?
FURTHER REASONINGS:
Lloyd: In my previous reasonings I cited bias as the reason I included Lloyd over Heihachi. However, looking back at what Sakurai said about Heihachi, I don't think we'll be seeing him in this game. When asked about Heihachi, Sakurai said "However, implementing Heihachi’s movement in Smash would be difficult." I think what's significant here is that he's not citing lack of a moveset, but difficulty of implementing the movement of Tekken, which is a 3D fighter. The 3D aspect is important to the series, and I don't think Sakurai and his team are going to be able to emulate to a point they'll be satisfied. Instead, Lloyd presents himself as a 2D fighter (no free run in Tales of Symphonia) with an already viable moveset (dual wielding swords) and louder fan demand than Heihachi. If we get another Namco rep, it's likely going to be Lloyd. We could also get Heihachi alongside Lloyd or as DLC though.
Simon: I legitimately think that if Simon gets in Konami had something to do with it. There are a couple of leaks that state that Konami specifically requested Simon, or even held Snake hostage and forced Simon in for Sakurai and his team to use Snake, and knowing Konami nowadays it sounds plausible. Castlevania has also been seeing a recent push from Konami in the form of the Netflix series, which has a second season coming out this October. The show's been in development for 10 years, and was likely picking up steam behind the scenes when Sakurai approached Konami to use Snake. Dude's also iconic, and is really the last true classic third party we need in my opinion. Plus, dude also showed up in a fighting game with Snake and Bomberman, so I feel like we'll be seeing him in some form.
REVEAL IDEAS/PREDICTIONS:
I'm actually going to try to predict when/how these will be revealed. This isn't a leak and I'm not a prophet. I'm just having some fun.
K. Rool and Dixie Kong: Trailer in late July/August in a Donkey Kong themed trailer (surprise).
Incineroar: Either EVO, a random reveal trailer (a lá Lucina and Robin), or a special Pokémon Direct around the time Lets Go comes out. Incineroar bodies Little Mac after he wins a Championship Belt.
Geno, Simon, and Dark Samus: Probably somewhere around Halloween. Preferably a continuation of Ridley's trailer where Zero Suit Samus is fighting Ridley, only for their fight to be interrupted by Simon, only for that fight to be interrupted by Dark Samus, only for that fight to get interrupted by Geno, who takes them out and revives Mario and Megaman. Good guys win. Yay.
Isabelle, Ashley, and Bandana Dee: A special Smash Bros. direct (in December-ish) where they're highlighted in Mini-Trailers. Sakurai can make more Waluigi jokes about how these characters got promoted from assist trophies and enemies to playable while he didn't.
Chrom and Erdrick: Reveal trailer at some point. Has Lucina getting her ass handed to her by Erdrick, only to be intercepted by Chrom, who finally gets his chance.
Lloyd: End of the final Smash Bros. Direct (December-ish). Cloud, Erdrick, and Mario are getting the snot kicked out of them by something (Skeleton Dancer maybe?). Lloyd jumps in with Guardian to deflect the final blow and saves their asses.
Chorus Kids and Chibi-Robo: not revealed, left to be unlocked by the player. The other echoes can potentially be "revealed" like this as well.