Absolutely Returning:
Jigglypuff and Ness are from the original twelve smash characters, and therefore will not be removed from this game, I can basically guarantee it based on that fact. Falco is going to be making a return, likely with a moveset closer to Brawl than Melee once again with the way that his shine works and whatnot, but not going into free fall after using his side b? Hey, welcome to the recovery field, Falco. Basically, he's been in two games now, and on top of that, he's been a fan favorite. Ganondorf is the main antagonist in LoZ, so I think it would be silly of them to remove him as a character especially since he's also a fan favorite. Think of it this way, if people are hoping for Ghirahim and whatnot, then good luck getting him without Ganondorf in there first.
Probably Returning:
Game&Watch as well as Ice Climers have been in two of the three smash games so far. Game&Watch is just a hilarious character to me, and I think he's going to be coming back since Nintendo seems to be compiling all of their retro characters in one game, meaning that R.O.B. is probably going to be making a comeback as well. I feel like Sakurai wasn't just thinking of Mega Man, Sonic, and Mario when he was making Pac-Man playable. He wants to see a match between Game&Watch, Pac-Man, R.O.B., and Ice Climbers, a whole bunch of really retro characters.
Meta Knight is probably going to be coming back as well since he is the main...well, what exactly do you call him? Sometimes he's the main villain, other times, he ends up helping you through it all and pretending he's doing it because he has to. Nevertheless, that's besides the point. He's a staple character, much like King Dedede, so I think him and his sword are coming back.
Wolf O'Donnell is probably going back. I still think that Nintendo has no intention of cutting villains from the cast, and Wolf was so cool to me when I saw him as a playable character in brawl. He played kind of like the other spacies, but he was also unique. I thought he was cool, even if he was a clone, so whatever.
Wario is the best. That's enough said, really. He's the most hilarious character in the game in my opinion, especially when the Wario Waft is fully charged. Who else would be so hilarious? But anyways, once again, why would Nintendo cut the villains from the game? I mean, Wario is like the Jesse/James/Meowth of the Mario realm. He's not the worst villain, just the slapstick kind of guy and weird and just kind of shows up when you don't want him to. I think he's in, but who knows, I suppose.
Not Sure:
Lucas I'm not so sure about. He's probably in, I would think so at the very least. The only reason I put him down here in this section of "Not Sure" is because I don't know his series as well as I know the other series of Smash. I don't know how much of a staple character he is or how much he did in his games. I'll be honest, Earthbound and Mother never grabbed my attention as a kid, so I can't really vouch for him.
Probably Not, But I Guess:
In honesty, I wanted to throw him down in the section below this one. Mewtwo I honestly see not coming back. I wouldn't be disappointed if he was, but really don't think so. We have Pikachu, Lucario, Jigglypuff, Greninja, and Charizard. I don't think another character is due in my opinion, especially one that was cut out for a game. Sorry Mewtwo fans, but I am not at all with you on this one.
Not Happening:
Alright, so the pokemon, you already know why they aren't coming back. We don't need all of them. We already have a water type, so go away Squirtle, and we already have Pikachu, so bye bye Pichu. I don't think this game can handle any more Pokemon for this round. Maybe the next one, because people get really hype about pokemon. If they add something, it sure as hell won't be Ivysaur. Bye.
Snake is a Konami character. Konami funded and helped with the last game. Konami is not funding or helping with this game. Ergo, Snake is probably not going to be returning. Hell, even Konami said that he probably won't be coming back. I don't see why he should, since their company has no faith in it.
Roy. While you are our boy, you cannot come back into this game. We have four Fire Emblem Characters. That's DOUBLING the amount we had the first and second go around. It just wasn't popular back then. But Awakening was such a huge hit in the Western Hemisphere and whatnot that they added two characters from it. One of which is a direct clone with different hitbox data. Roy was a clone with different hitbox data. Bye Roy, sorry to see you stay gone.
Young Link. Do I even have to defend you? Is there anything I can say to make people believe you SHOULD be in the game? You were replaced. That is it, really simple. You were replaced by Toon Link, and I like him more anyways, if only because of my bias and how much I love Wind Waker. See ya...still.
Alts:
I would love to see an alternate costume for Ganondorf that was Ocarina of time Ganon. I think it is possible too, especially with the Little Mac shoutouts to the retro look of wireframe. I think it would be a really cool look and something that people wouldn't hate on at all. I love the retro shoutouts in games, so this is totally welcome.
Dr. Mario or Dr. Luigi would be totally cool as well. I'm not saying that it's probable, I'm just saying that I would like it. Whatever, this section isn't even speculation, it's just personal desire, haha.