There seems to be much debate surrounding the possible appearances and nonappearances of certain characters in Smash 4 as I can see from all the Smash threads. (Yes, I've read all 6 threads in their entirety) I think a few people have already done this but I am going to go over several characters that people seem to constantly bring up. Some mentioned here are probably guaranteed but I go over them anyway. The following speculation is my opinion, not fact. (Although I would like it to be)
I know that some of you have brought up this whiteboard.
I personally think every Brawl veteran on this whiteboard will remain in Smash 4 for obvious reasons. I mean, why put them on there in 2012 if they won't be in?
The original 12 will always be in Smash, even Jigglypuff.
In order of likelihood: Will, Probably, Might.
Wario - He's going to be in. Ashley is already represented in Smash 4 and why have her without the representative that first appeared in Brawl.
Sheik - She will be in. Sheik has been in 2 Smash games already. Although maybe the Phantom is Zelda's down-B and Sheik is separate, we just don't know. That might happen for both Zelda and Samus, their transformations might become separate characters.
Zero Suit Samus - She will be in. We've already seen the Zero Laser and it remains to be seen if Samus will transform or if ZSS will be standalone.
Meta Knight - He will be in. Meta Knight was in the Brawl E3 trailer. He was revealed befoer Dedede I think in brawl. Why have Dedede in Smash 4 but not Meta Knight? He will most likely be nerfed. (lol)
Falco + Wolf - Both will probably be in. Fox and Falco have been in for a while, it might be easy to port characters from Brawl. Wolf is rather difficult to explain for his possible inclusion.
Pokemon Trainer - He will probably be in. Personally I believe they will have Pokemon Trainer then a Hoenn or Kalos Trainer. Charizard is very iconic and Game Freak has made him shine with Mega Evolutions.
Lucas - He will be in. Nintendo has made recent efforts to make the Mother series relevant outside Japan. Earthbound finally came out in Europe. They even made a Miiverse community for Mother news...which hasn't been active in a while. GBA games are finally going to be on Wii U and I think they will put Mother 3 on it in English by E3. Lucas will probably be revealed next month as Mother 3 came out in April. It would be weird for him to be shown before Ness, however.
Ike - He will probably be in. Fire Emblem has become more popular in the West these past few years. (Despite that I've only played Sacred Stones) Some might question his possible inclusion in Smash 4, but I believe Sakurai said he wants no cuts fro Brawl. I do find it odd that Roy wasn't in Brawl even though he's in the files I think. I hope Ike and Roy will both be in Smash 4.
R.O.B. - He will probably be in. Nintendo's popularity in the start of their venture into video games started with the NES which had R.O.B. bundled. He's here to stay.
Now for some characters that haven't been playable.
Mii + Pacman - They will both be in. These 2 have actually just been discussed so I don't need to make a case for them. We all know the "rumor" which will eventually be proven true. No one guesses Wii Fit Trainer.
Palutena - She will be in. Kid Icarus has been revived and it was showcased in the Smash 4 reveal. All the franchises in the reveal have more than one character in Smash except for KI. We also have a "leak" that occurred a while ago that might further prove Palutena to be in Smash 4. Sakurai even mentioned how great it is to have a lot of women in Smash 4. Not to mention that stage with her statue.
Ridley - He will probably be in. Hoo boy... We all know Sakurai is a gigantic troll with this one. In an old Nintendo Power (RIP in peace) Sakurai mentioned he could be in the game but "he'd have to be a little slow, is that okay? (laughs)" It's time Metroid had another rep, and in fact villains are featured in the 3DS eShop, including Ridley. Wily is there too...why is that relevant? Mega Man is in Smash. Oddly enough it's Ridley's NES sprite but it makes sense because clicking the banner leads to the original Metroid.
Takamaru - He might be in. I'm sure we all know that in Nintendo Land there's a minigame with this franchise. (I have no idea about this franchise much) Just like with Sakurai putting Pit in Brawl, I think Nintendo is trying to make more games in his series. I believe there's also a trademark for a VC game, I'm not sure on that one. Sakurai is known for putting "retro" characters in Smash including Ice Climbers, G&W, and R.O.B. So Takamaru might fit the bill for this in Smash 4.
And one more:
Mewtwo - He might be in. Game Freak has included 2 Mega Evolutions for Mewtwo in XY. I think Sakurai might want to put Mewtwo back in Smash 4 after a long hiatus. We haven't seen anything relating to Mega Evolution in Smash 4 so maybe it could be his Final Smash.
EDIT: Of course I forgot someone.
Snake - He will probably be in. If Sonic can make it in, so can he. Some might argue that Snake only has a few games on Nintendo platforms unlike Sonic who has a lot. (Twin Snakes, MGS3D) But Kojima is good friends with Sakurai, and I just don't see him being cut. A lot of people seem to believe Snake was a one time deal as people thought with Sonic, but I don't think that's the case.