Honestly, I think Mewtwo's the only one of them with a significant chance. He's still highly requested, still relevant nowadays after getting a signature move in Gen 5 and
two Mega forms in Gen 6, and they could easily rework his moveset the way they did Pit's. Arguably, his biggest competition is Jigglypuff, since I kind of doubt they want 2/3 of Pokemon's reps to be from its first game.
I could see Dixie happening, though. We've had a couple nods to Donkey Kong Country already, with Kremlings in Smash Run and all of these unconfirmed Chunky sightings. If the roster's 51 characters, I'm pretty sure we'll get a new DKC rep. I'm hoping more for K. Rool, but Dixie would be okay.
I think a lot of people are actually
overrating Roy's chances. Being in Melee doesn't give him any special treatment (isn't that right, Mewtwo?), especially since he was a full on clone. If the developers are pushing for unique characters, having red hair instead of blue wouldn't keep Roy from feeling any less redundant than Chrom. And the biggest strike against him: there are other, arguably
more worthy Fire Emblem characters that have come into existence since Brawl. Awakening is actually
far more popular in Japan than Binding Blade, believe it or not.
And honestly...I don't get the hype for Roy. It seems like most (but not all) of the support for him comes from people who've played Melee but haven't played a ton of Fire Emblem games and don't realize how many other viable characters there are. Nostalgia isn't everything, you know?
As for the others: Dr. Mario would have to be an alternate costume for Mario, which could certainly happen. There might not be a new Zelda rep at all this time, much less half-clone versions of Zelda and Sheik. And if Gen 3 wasn't relevant at all until earlier this year, with development starting only a few months prior, there's no way we'll get a rep from then, much less ones as disliked overall as Plusle and Minun. I could see a Gen 3 mon being DLC, though, but something like Sceptile would be more likely.