Usually only lurk around this site, but since this game seems like all I can talk about, I'll just go ahead and post as well as reading.
Here's the roster I've developed, with explanation below.
Keep in mind it's based both on what I want and what I perceive to be likely.
Mario series: no newcomer. If there was a newcomer, I'd bet on Toad, but there's plenty of characters I prefer to bet on.
DK serier: K. Rool for reasons we are all aware of. No Dixie due to, imo, similar moveset fighting style to Diddy or potentially a light-weight clone of DK.
Wario Ware series: Here's my first big justification. The Wario Ware series has gone on to become Nintendo's go-to way to showcase hardware, and given that I see it as a relevant franchise in smash. As for, why Ashley? I'm basing it on both ease of making a moveset for her due to what's established in the series, and the fact she seems to be the more popular suggestion from people who want a Wario Ware rep. However, that said, this is one of my more risky and unlikely guesses.
Metroid: Obviously Ridley for reasons we all know.
Earthbound/Mother: I see Lucas coming back as the present veteran, Ness as one of the first unlockables, and Porky as a new unlockable.
The reason for Porky is that Nintendo seems to have realized the meaning this series has to fans, and thanks to the VC release and the fanfare it received, the fanbase will no doubt expand. As a series, it is currently more relevant than even Star Fox. And a way to honor that is to bring in the character that connects both Mother 2 and 3.
And it's not just a case of "oh, he's a villian and has appeared in both games." But really, just the fact that Porky is such an embodiment of the series, even more so than Ness and Lucas who are silent protagonists. Porky is tragic, depressing, funny, absurd, creepy ... soo many adjectives that really bring light to the fact of why Mother's tone is so unique and captivating.
Yes, I concede that he has a harder moveset potential than, say, Masked Man or Kumatora or Duster ... but he's just so much more important in the greater context of the series. The most likely scenario to me is no new Mother rep, but if there is one, I'm betting on Porky.
Zelda: Ghirahim replacing Toon Link. I think this series is represented excellently by the Triforce and having another link always feels excessive. Ghirahim brings something new to the table by being a villian outside the triforce who was a serious threat.
That said, this is easily my least desired character in my roster, and he's only there because I don't see the Zelda roster going down, and of all Zelda characters he makes the most sense to me. Tone-wise, Impa would be obvious, but the moveset potential for her would just mean striping Zelda of Sheik to avoid redundancy.
Takamaru: Most likely retro callback to me. Nintendoland and Samurai Warriors have increased awareness of this guy, and he's to Link what Pit is to Samus. I'd be surprised not to see him in and then hear about a revival of the franchise from Platinum Games or someone else.
Kirby: perfect trio is already in. Bandana Dee would be, imo, the most likely Kirby rep, but definitely not essential to the roster. Perhaps more fitting to be an assist.
Kid Icarus: the heart of Uprising is the rapport between Pit and Palutena. I can't see a better Kid Icarus rep, even though more than half of the cast would make a solid character addition. Also, I'm biased in that Palutena is by far my most desired character.
Starfox: this franchise is really currently frozen, and I don't see Krystal being relevant enough to be included. Even Pygma means more to the series, imo.
F-Zero and Pikmin: nothing new here, carry on ...
Fire Emblem: another one bound to be complicated. Marth isn't going anywhere, and I don't see Ike leaving either. Roy I don't see coming back.
Now, I used to believe Chrom was obvious, but after Sakurai opted for the Animal Crossing Villager instead of Tom Nook, commenting he's the one you play as, I've begun to believe more in the Tactician/Robin. Along with the fact that both Chrom and Lucina's moveset would obligatorily have swords and this series has more than swords. Robin, meanwhile, relies more on magic, and his/her moveset would as a result be much more unique. And really, it'd be nice to have a non-blue haired Fire Emblem rep.
Like Ashley, I see this one as a bit unlikely, but I'm sticking with it.
Pokemon: While I'd like a new trainer, I don't see it happening. I don't see Lucario happening either, like Roy to me he was more of a flavor of the month thing. Mewtwo to me is certain, and I imagine his useless Down B being replaced with the awakened form transformation in some way. Jigglypuff is now a mainstay to me.
Punch-Out!: Little Mac is happening. I don't see how it won't. Moving on.
Pac Man: My only wanted/expected third party rep. Pac Man is just waay too iconic not to make it, and Sakurai's interview with IGN implied he had been wondering about including a Namco character. Sonic being in Brawl made noise all over gaming sites due to the sheer meaning of the fight ... two gaming titans going at each other. Pac Man can make such noise to much greater degrees.
The moveset is easy, Namco would love the exposure, the character fits in perfectly as it has in Mario Kart, and it would lead to a game that is truly an all-star fight between the most iconic images in gaming. The way I see it, few other third parties deserve this spot over Pac Man, and NO others would have the sheer impact he would.
Ok, that's all! My apologies for the length of my explanations