Considering that Sakurai just revealed the stage select screen with all 103 stages, and stated that every stage in the game is available right from the start, I'd say those 103 stages are most definitely it. And okay, so maybe there's a stage or two--maybe even three--stages that we kinda wished were added in. But again, I'm not complaining.
Of course, if this is the case, then we can pretty much disconfirm every single newcomer prospect coming from a potentially brand new franchise in the base roster. That means if Vergeben is to be believed, then four unique newcomers is the most to expect (because we already hit five newcomers this morning, and neither of them included Isabelle or Incineroar--meaning Echo Fighters do not count). And every newcomer henceforth--may they be unique or an echo--will have to come from a franchise already represented. This leaves us with an extremely limited window of possibilities coming from:
1. Mario: Off the top of my head, literally three different candidates in Paper Mario, Captain Toad, and Geno. And I'd wager it's Geno.
2. Donkey Kong: We just got King K. Rool. All that remains are Dixie Kong, Funky Kong (as an Echo), and Cranky Kong.
3. Legend of Zelda: The only likely candidates are Impa, Skull Kid, and...Tingle. And honestly, based off the Assist Trophies, I'd say Skull Kid has the best shot. Though Impa could still make it in as an Echo Fighter (since, you know, she taught Sheik everything she knows and all).
4. Metroid: We just got Ridley and Dark Samus. The only possible candidate remaining is Sylux, and I doubt he's making it in.
5. Yoshi: With the return of the Mario 3D World stage, I think Kamek is now officially disconfirmed. So really, nobody.
6. Kirby: Within reason, the only likely candidate to be added into Smash Brothers is Bandana Dee.
7. Star Fox: With Krystal as an Assist Trophy, that pretty much leaves us with Slippy Toad. Even if you like the idea now though, what are the odds...?
8. Pokémon: Countless. But mostly from Gen 7--with Mimikyu's disconfirmation, that just leaves us with Decidueye, Lycanroc, Incineroar, or maybe Necrozma as a representative. With Vergaben leaning more towards Incineroar.
9. Earthbound: Ninten as an Echo Fighter of Ness. Maybe Porky. Maybe Claus as an Echo of Lucas. But I doubt it.
10. F-Zero: Either we get Black Shadow as an Echo, or nothing. Since we haven't had a new F-Zero game in roughly 15 years, I'm not counting on it.
11. Ice Climbers: Come, now. From this moment forward, any single character retro franchise is going to be given no comment.
12. Fire Emblem: We just got Chrom as an Echo. Who was the recent Fire Emblem lord again? If not him, or Celicia, then we'd likely will have to wait until DLC.
13. Game & Watch
14. Kid Icarus: We haven't had a new game in this series since the 3DS launch--and it's very hard to come by, btw. And based off that, we only really have Medusa or Hades left to look over.
15. Wario: Ashley is still an Assist Trophy. Sorry, guys. Honestly, I don't even know who else could join.
16. Metal Gear: Konami has already overtaken all other 3rd Party reps with 3 at this point. Be as it may, Raiden is the only possible choice at this point.
17. Sonic: Shadow as an Echo Fighter. Or Tails as a unique fighter. Leaning more towards Shadow as an Echo Fighter.
18. Pikmin: I know there are other options. But considering that Alph is still just an alt for Olimar, what can you honestly do with said characters?
19. R.O.B.
20. Animal Crossing: Isabelle as either an Echo or a joke fighter (maybe her main differences are lack of tools, and ability to heal damage to herself and others). The only other candidate is K.K. Slider as a unique fighter. If Vergeben is to be believed, then we're getting Isabelle.
21. Mega Man: All the good choices are now pretty much taken. So nothing.
22. Wii Fit: Literally nothing.
23. Punch-Out: Little Mac has several adversaries to choose from--mostly recurring. None are likely. Though I wouldn't mind seeing Mike Tyson as our newest representative--even though he's technically 4th party.
24. Mii Fighters: We didn't get any new Mii Fighters added in, so we aren't getting anybody. Hope our choices in customization are good.
25. Pac-Man: Literally the only other character that could work is Ms. Pac-Man as an Echo Fighter (shouldn't that be Mrs.?). But how would you differentiate her from Pac-Man?
26. Xenoblade: Maybe Elma. Possibly Rex. You never know with Sakurai. Though if I have to take a guess, it's probably going to be Rex.
27. Duck Hunt
28. Street Fighter: It's pretty much just Ken as an Echo of Ryu (even though they were designed to be exactly the same originally), or Chun-Li as a unique newcomer. Personally, I'd lean more towards Ken.
29. Final Fantasy: This one is kind of interesting. One can either stick with Final Fantasy VII as a source; pick up the main characters of other Final Fantasy games (most likely Noctis in that lineup); or settle for an Echo Fighter with Zack. I'd lean more towards Zack as the likely choice.
30. Bayonetta: Really, with Rodin as an Assist Trophy, and Jeannine's color scheme literally being reused as a Bayonetta alt, I'd say nobody.
31. Splatoon: Even though the Octolings are the only other possible candidate to add into Smash (albeit as an Echo), how would you differentiate them from the Inklings?
32. Castlevania: I think we've got everybody out of that franchise. And I mean pretty much everybody.
So really, we have 32 franchises to choose from to figure out who our remaining fighters are going to be. Of those 32, Yoshi, Ice Climbers, Game & Watch, Wario, R.O.B., Wii Fit, Mii Fighters, and Duck Hunt are the only franchises that have literally no other candidates left to offer, leaving us with 24. Let's also assume that the following franchises' chances are so miniscule at this point they might as well be dead or at least all but completely exhausted or irrelevant (for now): Metroid, Star Fox, Earthbound, F-Zero, Fire Emblem, Kid Icarus, Metal Gear, Pikmin, Mega Man, Punch-Out, Pac-Man, Bayonetta, and Castlevania.
Even if you believe any of these 13 franchises could still get more newcomers (and really, they're not completely out of options), they seem so bloody remote at this point that it's kind of pointless to talk about them lest we get really spoiled (besides, I already singled out Fire Emblem as getting a newcomer via DLC; and we got two Metroid newcomers, so let's not spoil ourselves here). Honestly, Fire Elmbelm and Kid Icarus probably have the best shots of either of these franchises at getting newcomers since they are all at least rich with history to explore.
But in all fairness, that pretty much leaves us with 11 franchises represented (out of 32) that have a probable shot at newcomers. In fact, a very good shot. Even if they may not be able to provide us with a unique fighter that people have wanted, at least we may be able to get an Echo Fighter. And in fact, of those 11, Mario, Donkey Kong, Zelda, Pokémon, Sonic, Final Fantasy, and to a lesser extent, Street Fighter probably have the richest pools of unique fighters to draw from. And while Kirby, Animal Crossing, Xenoblade, and Splatoon don't have a whole lot of potential fighters, still have that one or two strong candidates to draw from.
As such, the most likely candidates to be joining the battle are going to come from the following pool: Captain Toad, Geno, Paper Mario, Funky Kong, Dixie Kong, Cranky Kong, Impa, Skull Kid, Tingle, Bandana Dee, Incineroar, Decidueye, Lycanroc, Necrozma (heck, any Gen 7 Pokémon not yet disconfirmed, as well as any Pokémon that received a ton of votes in the Ballot), Isabelle, K.K. Slider, Elma, Rex, Octolings, Shadow, Tails, Ken, Chung-Li, Zack, any major FFVII character, or any Final Fantasy protagonist.
Now for the 3rd Party reps, let's just reduce our potential candidates to just one Echo Fighter each (leaving us with Shadow, Ken, and Zack to look through), and with Pokémon, let's just put a pin on there. We are still left with 15 likely reps to sift through. And again, this is assuming just Echoes off of 3rd Party reps, and excluding the Pokémon reps since there are still so many. But of the likely Pokémon reps, that still leaves us with 19--almost 20 characters--to choose from.
So, who do I honestly think are the best bets at getting in? Well, right off the bat, I think it's going to go to Geno, Skull Kid, Bandana Dee, and Incineroar. And keep in mind; I'm only sticking with the unique fighters. I think everybody else will either have outside chances, or will be relegated to Echo Fighter status (and no, I doubt Paper Mario will be an Echo Fighter--unless the idea is to base his moveset entirely around classic N64 Mario).
Add to that the fact that Geno, Skull Kid, and Bandana Dee all have massive fan support from the beginning, are all from established Nintendo franchises (granted, one of them is owned by a 3rd party company), and are very much still standing, with Skull Kid currently standing as the likeliest to get confirmed (thanks to that Moon Assist Trophy). Incineroar is the only one I cannot point towards the ballot for obvious reasons of being too recent; he's pretty much just in because of Vergeben's leak.
Speaking of which, Isabelle, and any other potential fan favorites would all get in as Echo Fighters (specifically Impa as an Echo of Sheik; Dixie for Diddy; and Shadow for Sonic).
The next likely candidates in place of characters like, say, Bandana Dee, would have to be either Elma or Rex (but not both); while the Octolings would have to be either an Alt to the Inklings, or an Echo Fighter. Though they are just outside. The reason I say this? Honestly, similar to Pikmin, I have no idea how one could differentiate the Octolings from the Inklings. And I think if either Elma or Rex were going to get in, you'd think we'd get a brand new Xenoblade stage that might hint at this. But we are not.
The only possible exception--a brand new franchise representative in spite the lack of a new stage--might actually be the Monster Hunter due to that boss monster/Assist Trophy in the Direct. But even then, this doesn't look like a selectable stage so much as it is a boss battle for a new Adventure/Subspace-like mode. In fact, in the Direct, it is shown to only ever feature one fighter fighting him at a time as a boss. And the Monster Hunter stage doesn't even appear in the Stage Select Screen.
But I do leave it out as a possibility that the Monster Hunter might potentially be a new playable character from a newly represented franchise. But outside from that, it's pretty much limited to just the main 32 represented franchises, specifically (and most likely) from 11 pf them.
Boy, I said a lot. Hopefully, I don't get in trouble for this. Just let me know if I did something wrong so I may correct it, mod. At the very least, I doubt I used any swear words.