Newcomers
Geno, Isaac, and Paper Mario really need no introduction. They've been some of the biggest Smash requests for almost a decade's worth now. I'm confident enough to say that all three of them have a plausible shot to finally make their way.
Skull Kid isn't because of muh chairs, it's solely for the MIA Assist and the fact that the Moon is now an Assist trophy. Skull Kid is associated heavily with the moon, his lack of appearance or even not making him use the moon is mega suspicious. Also I have a gut feeling that the whole strew of villain slideshows will continue past K. Rool, starting with the same reel including K. Rool.
Porky is the most unique main antagonist you could grab out of Mother. His spider mech (or configured bed mech if you so choose) allows him to use way more diverse attacks that aren't on the same level of Giy "how the **** do we use this still NES sprite" gas and Claus. Also villainy or something.
I'm still confident on Mach Rider because of the rifle used by Sakurai. Geno's hand gun only has the barrel to take note of or can just base off of his Wii U costume and refine upon it. To create an entirely new rifle for something (which we haven't seen for an item yet), Mach Rider is still up there IMO since the rifles have similar builds to them.
Chorus Kids were alluded to by Gematsu and files in the game itself show a Rhythm Heaven character was playable. Chrom was also in the same boat before getting cut in 4 and he finally showed up. Perhaps it'll be a convenient time to revive him, not for the sake of polls, but just because now he can.
Rayman is the go-to choice for Ubisoft rep, first choice, not even a ****ing question. He does have history with Nintendo and Ubisoft had always gotten Nintendo's back, even in the Great Depression Wii U era they still had them. Since they're collaborating more, I wouldn't be surprised if he showed up.
Finally, Steve. You probably already know my stance on him. In the distant year of 2015, I always had the belief that Geno wasn't going to be in the game as long as Cloud existed. And then Cloud got in the game, and now I think Geno's ready. I have the same stance on Steve. Banjo is equal to him I believe in pretty much every aspect and is really just a Boomer VS Zoomer ordeal, but I think Steve edges out on this one. Sorry.
I FORGOT INCINEROAR ****
Yeah he's the most likely I believe, easy to base off of from concept art and as much as I love Buzzwole starters get the most attention.
Echoes
They're Newcomers too but they deserve their own time.
Dixie Kong is an easy model copy/paste with a bit more adjustments to the hair and whatnot. And since we don't see Diddy using his tail as often, it makes a new boundary for Dixie to shine. Donkey Kong's recovery can be used instead of Diddy's, but make the hitbox smaller. Young Link and Toon Link have a different effect from regular Link where the sword swings of their recovery swing in and hit constantly every time, while Link's isn't. The same can apply to Dixie to make her slightly different.
Shadow is MIA, an easy as heck model swap because his model is already in the previous games and it can be applied almost immediately after a few cosmetic touch-ups to make him more Shadow-y. I'd say this takes the least amount of effort out of any Echo in the game possible. Make his recovery Mewtwo's teleport and call it a night
Ken is the OG Echo, the OG Clone charater. I feel like Ken can be a simplified Ryu; one where he doesn't require any inputs and all of his attacks are balanced out rather than the weak basic and strong input command Ryu. Also fire, that's a thing. I think the only real differences come in Final Smashes and the animation for the Focus Strike.
I wasn't considering Felix at first, but after the Simon/Richter incident, I completely expect Isaac to get another buddy to flesh out Golden Sun a tiny bit more. I originally intended to put in Matthew and then I realized "wait they look the exact same". No real reason why he needs a placement, he just can.
Bottom Row
The bottom row is what I expect to come out of DLC characters in the future.
I'll be honest, I don't expect Elma to appear anymore in the base game. Maybe because I'm not very confident in her ballot results, or maybe it's because of Xenoblade 2 possibly overshadowing her and Sakurai wanted to wait. But I believe both her and Rex/Pyra will appear simultaneously to give Xenoblade some more representation. Rex and Pyra are a given considering how Sakurai loves the game and the characters themselves.
Spring Man from ARMS, if it was made noticed and developed early, was probably going to be in the base game, not gonna lie. Even if ARMS didn't sell amazingly well, the potential of the character and his gimmick being fluid range (as opposed to Simon and Richter's fixed range) could open up for opportunities.
Fire Emblem will probably get a new character. It's inevitable. Is it Zelgius? Is it Hector? Is it Edelgard for the ad slot #3? Who ****ing knows, anything could happen when this series tries to show its face and not get crucified.
Banjo, as much as I make my arguments against him, is probably going to be in one way or the other. I don't believe he'll get in through base, but I do believe DLC is an easy ticket. I'm not gonna call it a shoo-in, I'm gonna call it a complementary afterthought after the game was said and done.
Ayumi could be a substitute for a Phoenix Wright-based character if Sakurai cannot get the licensing or just wants to show off his weird fetish for older titles. I know with Mach Rider it'd make two retro characters which may be deemed unnecessary or just weird, but there hasn't been a real detective character yet and you this schoolgirl is certainly the biggest crowd caller for its own company.
Travis Touchdown. No More Heroes 3. Ubisoft. You get the point.
I also have a select pair of Echoes underneath them. I have this idea that they might release Echoes in series packs, like they select a certain series and they get certain stuff out of it (Mii Costumes, maybe a stage, new tracks, and Echo Fighters).
Ninten (Ness) and Giygas (Mewtwo) bump Mother up to a whopping 5 characters in my roster, but since it's DLC and such worthless effort it probably doesn't matter since it's optional, it's a doable love letter to the series.
I don't believe Black Shadow would be in base, hell I'd be surprised if he even showed up at all. But through DLC, Black Shadow's resurgence of somewhat popularity may have come a tad late for the base game, but I can easily see him show up as Falcon's Echo fighter.
Impa is most people's next best bet for a Zelda character, which is understandable if they doubt Skull Kid. I doubt that she'll be in the game, but through DLC she will probably get notified for her sizeable supporters and become a Sheik Echo. And then Hilda is just piss easy.