I feel like typing up a thing about what characters I think are and aren't likely. I'm kind of tired but unable to sleep so I might miss a lot of stuff or type things wrong but whatever, I'd like to start some discussion.
Mario
I think the only real choices, barring some kind of weird Sakurai choice that no one will see coming, are Bowser Jr. and Paper Mario. Bowser Jr. is, I guess, the "logical" choice because he keeps getting in Mario games and he's pretty much a core Mario character right now. Paper Mario kind of feels like a more "Sakurai" choice because he's not as obvious of a character and I feel like there are more interesting things they could do with him. I don't really feel right saying one is more likely than the other, but I don't think Mario is going to get any other characters in. Waluigi is a spinoff-only character and Nintendo clearly has no interest in doing anything with him, and Toads are still just generic non-characters that can be easily represented through Peach and have no real distinguishing abilities. I guess there's always the chance that Sakurai might bring back the cut Melee characters or something, though, so maybe Dr. Mario has a chance, but I doubt that will happen.
But all that is assuming there even is a new Mario character. I kind of assume there will be, but who knows.
I don't think Yoshi or Wario will get any new characters because they don't have anyone other than Yoshi and Wario. I've seen people talk about bringing in Kamek or the WarioWare game developers, but I think that would be reaching. Kamek isn't even in games anymore and none of the WarioWare characters actually do anything. Captain Syrup, too, is basically a dead character.
Donkey Kong
Obviously, if this series gets another character, it's going to be Dixie or K. Rool. We know that Dixie was going to be included in Brawl as part of Diddy, but she got cut and Sakurai didn't think she was important enough to include on her own. The question is how much concept work was done on her before she got cut and whether she got cut (was she just less important than the characters who made it into the game or was she not worth including at all?) I would still probably put her above K. Rool just because he pretty much hasn't been in anything in years. Dixie hasn't either, but at least she kind of got considered for Brawl.
But I can see Donkey Kong just not getting any new characters at all, especially since the newest game in the series didn't have any viable candidates.
Legend of Zelda
I don't think this series is going to get a new character unless Sakurai pulls something weird out (like how Toon Zelda was considered for Brawl). All of Zelda's core cast is already in Smash Bros. and the developers have shown that, outside of Sheik (who is just part of Zelda anyway) and characters from series like Fire Emblem, one-off characters aren't going to make it into Smash Bros. (so probably no Ghirahim or Demise). People used to push for Vaati, but he's pretty much totally irrelevant, and I used to push for Tingle, but he's so strongly hated in the west that Nintendo has kind of swept him under the rug. So yeah, if Zelda gets anyone it's going to be some oddball choice that no one has thought of.
Star Fox
This series already has a bunch of characters and its last game that wasn't a remake came out six years ago. I guess the only viable choice would be Krystal (because I'm not sure how you could add one of the other Star Fox or Star Wolf members and differentiate them from the three characters already in the game), but she hasn't been in a new game for a long time, and she only ever used her staff in one game anyway, so Nintendo might not even go that route with her. I can't see this series getting a new character.
People talk about how they want Wolf to get cut, but people here dug up those Sakurai interviews that show that he wanted Wolf in both of the first two Smash Bros. games at some point during their developments and that Falco getting in was just a weird circumstantial thing, so if anyone gets cut (which I guess could maybe happen if character resources are tight), it'll be Falco.
F-Zero
The obvious next pick is Samurai Goroh, since Smash Bros. consistently portrays him as the #2 F-Zero character (I don't think Black Shadow is as likely as some people have made him sound, he's only really important in the anime which Smash Bros. has only sort of acknowledged). But F-Zero is kind of dead right now and I'm not even sure if Nintendo has interest in reviving it, so I strongly doubt it'll get a new character.
Metroid
The only plausible choice is Ridley, but that might be a stretch. It's clear based on his comments in that one Nintendo Power interview that Sakurai never even considered Ridley as a playable character, but there has been a lot of fan demand for him. I think that this is going to come down to however Sakurai felt during the character selection process for this game - did the fan demand make him seriously consider Ridley or did he just go "nah"? Metroid doesn't have any other characters that could make it in - every other important character is some kind of monster who would be even less workable than Ridley, and all of the humanoid characters are either one-offs (the bounty hunters) or essentially one-offs (Dark Samus). Dark Samus would have been awesome, though.
Kirby
There are already a bunch of Kirby characters, the series' core cast is all represented, and I think Sakurai is still going to be humble about representing his own series (or at least he'll try to be), so I would bet on there being no new additions here. The bandanna Waddle Dee would be kind of cool to see, but I think that's about as likely as Toad getting in Smash Bros. for similar reasons.
Pokemon
I'm like 99% sure they'll put in a gen 5 Pokemon. I guess it would be Zoroark, since that's the current big mascot, but I'm not up on my Pokemon so who knows. I don't think they'll include another trainer character (for example, N, like a lot of people have said) because that would be encroaching on the current trainer's gimmick (although they might remove the old trainer, who knows).
I think Lucario is one of the most likely Brawl characters to get cut. I don't mean that I think he's likely to get cut, just that he's probably going to be lower-priority than most other characters since he mainly represents a few specific games that have already been superseded. (But since he is apparently still being used in promotional materials for the newest Pokemon games, I think he's likely to stay on as a new-gen Pokemon representative, and there is that 1% chance that an actual new Pokemon doesn't make it in, which would make Lucario a lock, I guess.)
Then there's the whole Mewtwo thing which is kind of complicated. I guess I would put his odds at 60%, but I might be being kind of generous. He's still a big icon for the series, he probably got cut from Brawl just because of time restraints and not because the developers didn't want him back, and I think a lot of people did complain about him not being in the game, so there are definitely a lot of reasons for him to make it back. But I don't know if there's going to be as much of an emphasis on a big roster this time, and there are definitely going to be a decent amount of newcomers who will probably be higher-priority than an older cut character, so I'm not sure we can say either way. It's definitely possible that Mewtwo could return, and he is by far the most likely cut character to come back, but I don't think I'd bet on it.
I don't really think the Pokemon Trainer or Jigglypuff's spots are secured. I'm sure that both of them won't get cut, but Jigglypuff is really just a joke character (who might not have even been a lock for Brawl) and the Pokemon Trainer took a lot of resources to develop and balance, so I think it's possible that one of them might get cut. I would hope that both return, though.
I also don't think there will be any second or third-gen characters unless Sakurai goes with Pichu or Plusle and Minun just because they'd be easy clones.
Mother
I love this series, really, but it's over and it doesn't need any more characters.
Mother 3 isn't as popular among Japanese gamers as the first two games (or at least it kind of seems that way - the first two games are beloved in a nostalgic kind of way, and the third game doesn't have that going for that), so I don't think Lucas's spot is secure. His moveset being sort of similar to Ness could help him stay (since he would be marginally easier to bring back than a 100% unique character) or get him cut (since a lot of people see him as a clone and have complained about him). Because Mother is such a small series, I can see Lucas getting dropped if roster space is tight and Sakurai feels Lucas representing the newest game in the series isn't important anymore.
Fire Emblem
Like Pokemon, Fire Emblem changes casts every game, and there is absolutely no way they won't represent the newest game in the series, so I'm confident that Chrom is a lock. There is no way he won't be in. I guess the big question is Ike. I think he's about as likely as Lucario - he's still fairly popular (enough to get some references in Awakening, and I think the default male avatar in the second DS game might have been based on him, but don't quote me on that), but his first game was one of the lowest-selling in the series in Japan and he's not likely to ever get a "proper" appearance again. I could see Ike's status going either way. I will say that I can't really see Chrom or Roy inheriting Ike's moveset because he's so much burlier than them, though.
Then there's Roy. I think Roy is less likely than Mewtwo, partially because Fire Emblem isn't as big as Pokemon and because Roy has stiffer competition (every Fire Emblem character is already going to be a swordsman, so it's going to be hard for Roy to stand out, and he would have to directly compete with Chrom and Ike, who are both more relevant). I think that if Roy gets in, it'll be as a Melee-style clone again. I know that there is a pretty big amount of fan outcry and that he was considered for Brawl (although he probably would have pretty much been a clone, it seems like the other cut characters were), but I don't think he's seen as a very high-priority character, and there are gameplay reasons he might not make it back too. I don't think the series will get four characters.
I also don't see this series getting any non-swordsman characters. There just aren't any who are relevant enough.
Pikmin
It's sort of early to make judgements on this, since I don't think we even know how Olimar will play into the new game. I guess there's a chance that they might use a character from Pikmin 3 to replace Olimar or enter the game as sort of a semi-clone of him, I haven't really kept up with that game so I don't know. I would lean towards Olimar being the sole character again, though.
Miscellaneous stuff
Animal Crossing very obviously "deserves" a character. Even Sakurai said that. But it seems like both Sakurai and the people in charge of Animal Crossing would rather not see characters from that series act as fighters in Smash Bros., according to that one Famitsu interview. On paper, Animal Crossing "should" get a character, but I don't believe it will.
People talk about Golden Sun, Sin and Punishment, Xenoblade, and series like that a lot, but I'm going to completely rip off another poster's argument and say that I think history has shown that "B-string" series like that don't get playable characters in Smash Bros. Not counting series added just to act as retro representation and the third-party series added in Brawl, the only new series added were Fire Emblem (in Melee), Wario, and Pikmin (in Brawl). Not only is Nintendo apparently really selective about new series representation, but they only stick to really big series that are closely tied to Nintendo (that aren't pretty much developed by other companies). I mean, I really want to see Saki and Shulk, but I don't think that's realistic.
If Kid Icarus: Uprising did well, then I think Palutena is guaranteed. She has immediately obvious moveset potential, she's from a Sakurai game, and she is a really major character in Uprising and its promotional material. I think it's pretty clear that no other character is going to beat her out (the villains have less screentime and presence than her in Uprising and Magnus doesn't have any unique abilities that a Fire Emblem character couldn't imitate). But if Uprising didn't do too well, then Sakurai probably won't go for another one of his characters.
Retro stuff
I think Takamaru is really likely. He's gotten several big references in games recently (the one in Nintendo Land is really surprising to me and I do kind of think it's indicative that Nintendo has some kind of plan for his series) and his original game seems like it enjoyed popularity akin to Kid Icarus or Punch-Out (in the west) back when it came out. I guess there's always the possibility that Sakurai might want to stay away from including too many Japan-only characters, though, so who knows.
Little Mac is another character that seems really obvious on paper (Punch-Out is a classic and it even got a recent game), but I think there are some big potential issues with him (Punch-Out isn't really big in Japan at all and Mac has basically no real moveset potential). I think he has a shot, but I think he's fairly far from being a shoo-in. I will say that I really like the idea that Mac and Takamaru could get in and sort of simultaneously appease both the US and Japan, though.
Then there are other potential characters like Balloon Fighter or some kind of Duck Hunt representation or whatever. This is probably the place where Sakurai would be most likely to pull out something crazy and unexpected, so it's harder to say what will happen here than it is for the established series.
Third-party stuff
I guarantee that Sonic will be back. Neither Nintendo or Sega will let this chance slip. Snake, I'm not so sure about. Metal Gear doesn't really have any strong ties to Nintendo, Snake only got in Brawl because Kojima begged for it and not because of fan desire or anything, he was a guest character, and a decent number of fans have
complained about him not fitting the tone of the game. I hope he comes back but I'm not sure I would bet on it. I will say that I like to think that the primary reason he wasn't in Playstation All-Stars is because Nintendo scored some kind of exclusivity agreement, though, but I doubt that.
I don't feel like speculation on new third-party characters is going to go anywhere. The only candidates are going to be characters who are really big for their respective companies (so I don't think we'll see, like, Bill Rizer because Contra isn't big compared to Metal Gear now), and I don't think characters who aren't huge icons in the gaming world will make it in (so I'm betting against Bayonetta and Travis Touchdown). I guess Pac-Man is fairly likely because of Namco's involvement, since, unlike GameArts, their involvement was announced early on, and Pac-Man is a huge gaming icon that Nintendo has used in the past. Other than that I wouldn't make any bets.
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I brought this up a few times, but for the most part, whenever I say "if Sakurai does something weird", I usually meant "if Sakurai decides to use clones and we get weird character choices like in Melee". So if they go that route, we might get, like, Dr. Mario, Samurai Goroh (as a clone of a Fire Emblem character or Takamaru or something), Toon Zelda, and other stuff like that.
The end.