My roster is mostly speculation, with a little bit of wishful thinking thrown in for good measure.
Mario, Peach, Luigi, Bowser: Fine where they are
Waluigi: Reps Mario spinoffs as a whole perfectly. I can see him with moves based off of Mario Party, Kart, Tennis, Golf, Strikers, Baseball, Hoops, Sports...and he can swim in midair. PERFECT.
Yoshi, Wario: Fine where they are, with none too many fellow series reps to join them. Well, iconic ones, anyway. Save for Kamek, who I would prefer other franchises over.
ROB: Too unique a character, with a rich Nintendo history. Flesh out his moveset and he's more than a keeper.
DK, Diddy: Stay put.
K. Rool, Dixie: Iconic villain vs. iconic female. They're both so iconic that I just put both in.
Link, Zelda, Ganondorf: They make up the Triforce. They're the biggest part of the franchise, in-game or not.
Sheik: A key part of one of the most famous video games ever. A huge part of what makes Zelda a unique character. I'd rather not see her go.
Toon Link: Link has gone through so many transformations that even his alternate looks have gotten a slew of games. Toon Link is one of the most iconic, as well as the most prevalent. Plus...Wind Waker HD. 'Nuff said.
Pikachu: The only Pokemon character who's really locked in, 100%.
Jigglypuff: While it's a stretch, Jiggly has been around since SSB64. Plus, the Fairy-type makes it relevant again. As if relevance matters.
Mewtwo: Sakurai has even addressed it. Mewtwo has enough demand. Plus, he's no stranger to the roster.
Pokemon Trainer: A unique character who wields the most famous of Pokemon. Charizard can't help his fame.
Lucario: The only Pokemon to rep a Gen other than uno (other than Pichu, but...no). A fifth rep is no biggy for a franchise as insanely huge and famous as Pokemon.
Kirby, Meta Knight, King Dedede: The main character, the bumbling-misunderstood-villain-of-sorts, and the fan-favorite topsy-turvy protagonist round out the franchise quite nicely.
Pit: No reason to take him out. Especially since we know he's in the game. Palutena, on the other hand, should stay on a god-like status. Maybe a codec of sorts? That'd be neat.
Ice Climbers: Fix the 3DS problems and there's no reason to leave them out.
Mr. Game & Watch: Nintendo's first major IP, and first "hardware". Plus, he has a wacky and specific moveset. There's no reason to drop him.
Takamaru: A retro character who has something unique to bring to the table. Plus, we could get the whole "East vs. West" thingy with him and Little Mac.
Samus: She's a mainstay who's already in.
Ridley: The only other available and understandable Metroid rep who isn't a Samus clone. And he isn't too big.
Ness: A 64 vet whose game recently got rereleased on the WiiU (at least in America).
Lucas: He has a potential moveset away from Ness. Give it to him, and viola.
Fox, Falco, Wolf: The same case I made with the Kirby series. The main protagonist, the sidekick-ace-pilot guy, and the hardened rival/antagonist type. The series has more than enough rep. Flesh out Falco and Wolf more and leave it be, from there.
Dillon: A unique character who has more than proven himself as a contemporary pseudo-All-Star, and the face of the 3DS eShop. There is no character I can think of that could bring what Dillon can to the table.
Marth: The original FE lord. He has been in more FE games than any other FE character. Melee and Brawl only have solidified him as the ideal rep for Fire Emblem.
Ike: Unlike Roy before him, Ike differed from his counterpart, Marth. He is the last console rep of FE. They could still play off of his fame from the "Radiant" games.
Chrom: Fire Emblem: Awakening has brought up a greater understanding of the franchise in the West. Who better to play off of that than Chrom himself?
Isaac: The face of Golden Sun whose Psynergy-based moveset would differ from regular swordsmen.
Captain Falcon: The famous face of a dormant series. I'd prefer to see a new franchise altogether than see Black Shadow or even Samurai Goroh.
Little Mac: Once a famous retro character, Brawl and Punch-Out Wii brought the character back into the spotlight. He's practically a shoe-in. Plus, Little Mac vs. Takamaru, as I've said before, would be a great representation of East vs. West.
Starfy: In the past few years, Starfy's presence has grown to be more recognized. He did get a DS release shortly after Brawl, the same thing that put Little Mac back on the radar (kinda sorta).
Olimar: The face of Pikmin, and a Brawl vet. Even if we weren't aware of his presence in SSB4 already, replacing him with a so-far one-time protagonist would be insane.
Villager: He's in. That's all I got.
Chibi-Robo: Since Brawl, his series has grown into a GC game, a DS game, a Japan-exclusive DS game, and a Japan-exclusive 3DS eShop game that came out recently. He is a, more or less, unique character. Even with his chances now boosted, he's mostly a wishlist character.
Jill: Once purely a wishlist character, after playing Drill Dozer, the only strike against her would be the fact that she's in one game. I think she'd fit right in with the Smash cast.
Wii Fit Trainer: Same as Villager, but to a much much MUCH higher extent.
Snake: Although he is pretty irrelevant to Nintendo, in the grand scheme of things, he was in Brawl, and he was pretty unique, so why not?
Sonic: More and more, Sonic has become an IP that shows faithfulness to Nintendo as a third-party rep. Especially with his exclusive Nintendo gig, and the fact that a Sega rep said that the only thing keeping Sonic from SSB4 was Sakurai's input, he could be considered to be a shoe-in, under a slightly optimistic light.
Mega Man: Doi.
Mii: Miis are a trademark Nintendo avatar with playable potential. And yes, they would work in a stat-based game, if Mario Kart and Mario & Sonic have taught us anything.
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